Sciencemadness Discussion Board

Claimed quantum computer breakthough

MadHatter - 6-12-2020 at 17:11

https://www.wired.com/story/china-stakes-claim-quantum-supre...
Food for thought. I don't know whether to believe it or not.

clearly_not_atara - 6-12-2020 at 17:18

Supposedly a reanalysis is being done with results expected in days to weeks. I'm pretty busy right now so I see no reason to speculate; I'll let them hash it out.

Fyndium - 7-12-2020 at 07:48

So when are the current encryption standards at stake?

Safety

MadHatter - 8-12-2020 at 18:46

https://arstechnica.com/science/2020/12/un-computable-quantu...
Just when you thought it was safe to explore anything.

leau - 5-12-2021 at 15:51

Connecting heterogeneous quantum networks by hybrid entanglement swapping

Giovanni Guccione, Tom Darras, Hanna Le Jeannic, Varun B. Verma, Sae Woo Nam, Adrien Cavaillès & Julien Laurat

Guccione et al., Sci. Adv. 2020; 6 : eaba4508 29 May 2020

Recent advances in quantum technologies are rapidly stimulating the building of quantum networks. With the parallel development of multiple physical platforms and different types of encodings, a challenge for present and future networks is to uphold a heterogeneous structure for full functionality and therefore support modular systems that are not necessarily compatible with one another. Central to this endeavor is the capability to distribute and interconnect optical entangled states relying on different discrete and continuous quantum variables. Here, we report an entanglement swapping protocol connecting such entangled states. We generate single-photon entanglement and hybrid entanglement between particle- and wave-like optical qubits and then demonstrate the heralded creation of hybrid entanglement at a distance by using a specific Bell-state measurement. This ability opens up the prospect of connecting heterogeneous nodes of a network, with the promise of increased integration and novel functionalities


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draculic acid69 - 5-12-2021 at 21:33

Quote: Originally posted by Fyndium  
So when are the current encryption standards at stake?


Now.or maybe soon. I wouldn't count on anything being unbreakable these days.

leau - 6-12-2021 at 10:51

Teleportation Systems Toward a Quantum Internet

Raju Valivarthi, Samantha I. Davis, Cristián Peña, Si Xie , Nikolai Lauk, Lautaro Narváez, Jason P. Allmaras, Andrew D. Beyer, Yewon Gim, Meraj Hussein, George Iskander , Hyunseong Linus Kim, Boris Korzh, Andrew Mueller, Mandy Rominsky, Matthew Shaw, Dawn Tang , Emma E. Wollman, Christoph Simon, Panagiotis Spentzouris, Daniel Oblak, Neil Sinclair and Maria Spiropulu

PRX QUANTUM 1, 020317 (2020) DOI: 10.1103/PRXQuantum.1.020317

Quantum teleportation is essential for many quantum information technologies, including long-distance quantum networks. Using fiber-coupled devices, including state-of-the-art low-noise superconducting nanowire single-photon detectors and off-the-shelf optics, we achieve conditional quantum teleportation of time-bin qubits at the telecommunication wavelength of 1536.5 nm. We measure teleportation fidelities of ≥ 90% that are consistent with an analytical model of our system, which includes realistic imperfections. To demonstrate the compatibility of our setup with deployed quantum networks, we teleport qubits over 22 km of single-mode fiber while transmitting qubits over an additional 22 km of fiber. Our systems, which are compatible with emerging solid-state quantum devices, provide a realistic foundation for a high-fidelity quantum Internet with practical devices.


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[Edited on 6-12-2021 by leau]

macckone - 6-12-2021 at 11:00

For reference purposes you need at minimum the number of qubits in a number to factor it.

They are at 50-60 qubits and RSA encryption is currently using 4096 bits for long duration keys and 2048 for normal use keys.
There is a good ways to go.
The NSA has recommended against ECC for a while now, so my guess is they have a shortcut for ECC at least with some curves.

leau - 7-12-2021 at 13:20

How to profit from quantum technology without building quantum computers

Dmitry Green, Henning Soller, Yuval Oreg and Victor Galitski

www.nature.com/natrevphys
150 | March 2021 | volume 3

There are a number of lower risk opportunities to invest in quantum technologies, other than quantum computers, but to make the most of them both specialist knowledge and market awareness are required.


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leau - 8-12-2021 at 08:33

Experimental Fock-state bunching capability of non-ideal single-photon states

Petr Zapletal, Tom Darras, Hanna Le Jeannic, Adrien Cavaillès, Giovanni Guccione, Julien Laurat & Radim Filip

Vol. 8, No. 5 / May 2021 / Optica

Advanced quantum technologies, as well as fundamental tests of quantum physics, crucially require the interferenceof multiple single photons in linear-optics circuits. This interference can result in the bunching of photons into higher Fock states, leading to a complex bosonic behavior. These challenging tasks timely require to develop collective criteria to benchmark many independent initial resources. Here we determine whether n independent imperfect single photons can ultimately bunch into the Fock state |ni. We thereby introduce an experimental Fock-state bunching capability for single-photon sources, which uses phase-space interference for extreme bunching events as a quantifier. In contrast to autocorrelation functions, this operational approach takes into account not only residual multi-photon components but also a vacuum admixture and the dispersion of individual photon statistics. We apply this approach to high-purity single photons generated from an optical parametric oscillator and show that they can lead to a Fock-state capability of at least 14. Our work demonstrates a novel collective benchmark for single-photon sources and their use in subsequent stringent applications.


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leau - 9-12-2021 at 07:44

Validating multi-photon quantum interference with finite data


Quantum Sci. Technol. 5 (2020) 045005

https://doi.org/10.1088/2058-9565/aba03a

Fulvio Flamini, Mattia Walschaers, Nicolò Spagnolo, Nathan Wiebe, Andreas Buchleitner and Fabio Sciarrino

Multi-particle interference is a key resource for quantum information processing, as exemplified by Boson Sampling. Hence, given its fragile nature, an essential desideratum is a solid and reliable framework for its validation. However, while several protocols have been introduced to this end,the approach is still fragmented and fails to build a big picture for future developments. In this work, we propose an operational approach to validation that encompasses and strengthens the state of the art for these protocols. To this end, we consider the Bayesian hypothesis testing and the statistical benchmark as most favorable protocols for small- and large-scale applications, respectively. We numerically investigate their operation with finite sample size, extending previous tests to larger dimensions, and against two adversarial algorithms for classical simulation: the mean-field sampler and the metropolized independent sampler. To evidence the actual need for refined validation techniques, we show how the assessment of numerically simulated data depends on the available sample size, as well as on the internal hyper-parameters and other practically relevant constraints. Our analyses provide general insights into the challenge of validation, and can inspire the design of algorithms with a measurable quantum advantage.


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leau - 10-12-2021 at 09:37

Teleportation Systems Toward a Quantum Internet

Raju Valivarthi, Samantha I. Davis, Cristián Peña, Si Xie, Nikolai Lauk, Lautaro Narváez, Jason P. Allmaras, Andrew D. Beyer, Yewon Gim, Meraj Hussein, George Iskander, Hyunseong Linus Kim, Boris Korzh, Andrew Mueller, Mandy Rominsky, Matthew Shaw, Dawn Tang, Emma E. Wollman, Christoph Simon, Panagiotis Spentzouris, Daniel Oblak, Neil Sinclair and Maria Spiropulu

PRX QUANTUM 1, 020317 (2020)
DOI: 10.1103/PRXQuantum.1.020317

Quantum teleportation is essential for many quantum information technologies, including long-distance quantum networks. Using fiber-coupled devices, including state-of-the-art low-noise superconducting nanowire single-photon detectors and off-the-shelf optics, we achieve conditional quantum teleportation of time-bin qubits at the telecommunication wavelength of 1536.5 nm. We measure teleportation fidelities of ≥ 90% that are consistent with an analytical model of our system, which includes realistic imperfections. To demonstrate the compatibility of our setup with deployed quantum networks, we teleport qubits over 22 km of single-mode fiber while transmitting qubits over an additional 22 km of fiber. Our systems, which are compatible with emerging solid-state quantum devices, provide a realistic foundation for a high-fidelity quantum Internet with practical devices.


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leau - 11-12-2021 at 07:14

Efficient Quantum Teleportation of Unknown Qubit Based on DV-CV Interaction Mechanism

Sergey A. Podoshvedov

Entropy 2019, 21, 150; doi:10.3390/e21020150

We propose and develop the theory of quantum teleportation of an unknown qubit based on the interaction mechanism between discrete-variable (DV) and continuous-variable (CV) states on highly transmissive beam splitter (HTBS). This DV-CV interaction mechanism is based on the simultaneous displacement of the DV state on equal in absolute value, but opposite in sign displacement amplitudes by coherent components of the hybrid in such a way that all the information about the displacement amplitudes is lost with subsequent registration of photons in the auxiliary modes. The relative phase of the displaced unknown qubit in the measurement number state basis can vary on opposite, depending on the parity of the basis states in the case of the negative amplitude of displacement that is akin to action of nonlinear effect on the teleported qubit. All measurement outcomes of the quantum teleportation are distinguishable, but the teleported state at Bob’s disposal may acquire a predetermined amplitude-distorting factor. Two methods of getting rid of the factors are considered. The quantum teleportation is considered in various interpretations. A method for increasing the efficiency of quantum teleportation of an unknown qubit is proposed.


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[Edited on 11-12-2021 by leau]

leau - 12-12-2021 at 07:54

Scheme for the generation of hybrid entanglement between time-bin and wavelike encodings

Élie Gouzien, Floriane Brunel, Sébastien Tanzilli, and Virginia D’Auria

Phys. Rev. A 102, 012603 – Published 2 July 2020
DOI:https://doi.org/10.1103/PhysRevA.102.012603

We propose a scheme for the generation of hybrid states entangling a single-photon time-bin qubit with a coherent-state qubit encoded on phases. Compared to other reported solutions, time-bin encoding makes hybrid entanglement particularly well adapted to applications involving long-distance propagation in optical fibers. This makes our proposal a promising resource for future out of-the-laboratory quantum communication. In this perspective, we analyze our scheme by taking into account realistic experimental resources and discuss the impact of their imperfections on the quality of the obtained hybrid state.


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leau - 13-12-2021 at 10:32

Small quantum computers and large classical data sets

Aram W. Harrow

https://arxiv.org/abs/2004.00026

We introduce hybrid classical-quantum algorithms for problems involving a large classical data set X and a space of models Y such that a quantum computer has superposition access to Y but not X. These algorithms use data reduction techniques to construct a weighted subset of X called a coreset that yields approximately the same loss for each model. The coreset can be constructed by the classical computer alone, or via an interactive protocol in which the outputs of the quantum computer are used to help decide which elements of X to use. By using the quantum computer to perform Grover search or rejection sampling, this yields quantum speedups for maximum likelihood estimation, Bayesian inference and saddle-point optimization. Concrete applications include k-means clustering, logistical regression, zero-sum games and boosting.


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leau - 14-12-2021 at 07:57

Highly photon loss tolerant quantum computing using hybrid qubits

S. Omkar, Y. S. Teo, Seung-Woo Lee, and H. Jeong

doi:10.1103/PhysRevA.103.032602 arXiv:2011.04209

We investigate a scheme for topological quantum computing using optical hybrid qubits and make an extensive comparison with previous all-optical schemes. We show that the photon loss threshold reported by Omkar et al. [Phys. Rev. Lett. 125, 060501 (2020)] can be improved further by employing postselection and multi-Bell-state-measurement based entangling operation to create a special cluster state, known as Raussendorf lattice for topological quantum computation. In particular, the photon loss threshold is enhanced up to 5.7 × 10 −3 , which is the highest reported value given a reasonable error model. This improvement is obtained at the price of consuming more resources by an order of magnitude, compared to the scheme in the aforementioned reference. Neverthless, this scheme remains resource-efficient compared to other known optical schemes for fault-tolerant quantum computation.


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leau - 15-12-2021 at 05:44

Quantum Spin Liquid States

Yi Zhou, Kazushi Kanoda, Tai-Kai Ng

Rev. Mod. Phys. 89, 025003 (2017) DOI: 10.1103/RevModPhys.89.025003

This article is an introductory review of the physics of quantum spin liquid (QSL) states. Quantum magnetism is a rapidly evolving field, and recent developments reveal that the ground states and low-energy physics of frustrated spin systems may develop many exotic behaviors once we leave the regime of semi-classical approaches. The purpose of this article is to introduce these developments. The article begins by explaining how semi-classical approaches fail once quantum mechanics become important and then describes the alternative approaches for addressing the problem. We discuss mainly spin 1/2 systems, and we spend most of our time in this article on one particular set of plausible spin liquid states in which spins are represented by fermions. These states are spin-singlet states and may be viewed as an extension of Fermi liquid states to Mott insulators, and they are usually classified in the category of so-called SU (2), U (1) or Z 2 spin liquid states. We review the basic theory regarding these states and the extensions of these states to include the effect of spin-orbit coupling and to higher spin (S > 1/2) systems. Two other important approaches with strong influences on the understanding of spin liquid states are also introduced: (i) matrix product states and projected entangled pair states and (ii) the Kitaev honeycomb model. Experimental progress concerning spin liquid states in realistic materials, including anisotropic triangular lattice systems (κ-(ET) 2 Cu 2 (CN) 3 and EtMe 3 Sb[(Pd(dmit) 2 ] 2 ), kagome lattice systems (ZnCu 3 (OH) 6 Cl 2 ) and hyperkagome lattice systems (Na 4 Ir 3 O 8 ), is reviewed and compared against the corresponding theories.


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leau - 16-12-2021 at 09:52

Probing Topological Spin Liquids on a Programmable Quantum Simulator

G. Semeghini, H. Levine, A. Keesling, S. Ebadi, T. T. Wang, D. Bluvstein , R. Verresen, H. Pichler M. Kalinowski, R. Samajdar, A. Omran, S. Sachdev , A. Vishwanath , M. Greiner, V. Vuletić , M. D. Lukin

DOI: 10.1126/science.abi8794
https://arxiv.org/pdf/2104.04119

Quantum spin liquids, exotic phases of matter with topological order, have been a major focus of explorations in physical science for the past several decades. Such phases feature long-range quantum entanglement that can potentially be exploited to realize robust quantum computation. We use a 219-atom programmable quantum simulator to probe quantum spin liquid states. In our approach, arrays of atoms are placed on the links of a kagome lattice and evolution under Rydberg blockade creates frustrated quantum states with no local order. The onset of a quantum spin liquid phase of the paradigmatic toric code type is detected by evaluating topological string operators that provide direct signatures of topological order and quantum correlations. Its properties are further revealed by using an atom array with nontrivial topology, representing a first step towards topological encoding. Our observations enable the controlled experimental exploration of topological quantum matter and protected quantum information processing.


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leau - 17-12-2021 at 07:08

Supplementary Materials for Probing topological spin liquids on a programmable quantum simulator

G. Semeghini et al.

Science 374, 1242 (2021) DOI: 10.1126/science.abi8794


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leau - 22-12-2021 at 08:37

Quantum Teleportation Between Discrete and Continuous Encodings of an Optical Qubit

Alexander E. Ulanov, Demid Sychev, Anastasia A. Pushkina, Ilya A. Fedorov, and A. I. Lvovsky

DOI: 10.1103/PhysRevLett.118.160501

The transfer of quantum information between physical systems of a different nature is a central matter in quantum technologies. Particularly challenging is the transfer between discrete and continuous degrees of freedom of various harmonic oscillator systems. Here we implement a protocol for teleporting a continuous-variable optical qubit, encoded by means of low-amplitude coherent states, onto a discrete-variable, single-rail qubit—a superposition of the vacuum and single-photon optical states—via a hybrid entangled resource we test our protocol on a one-dimensional manifold of the input qubit space and demonstrate the mappingonto the equator of the teleported qubit’s Bloch sphere with an average fidelity of 0.83 - 0.04. Our work opens up the way to the wide application of quantum information processing techniques where discrete- and continuous-variable encodings are combined within the same optical circuit.


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leau - 23-12-2021 at 07:43

Hybrid entanglement between optical discrete polarizations and continuous quadrature variables

Jianming Wen, Irina Novikova, Chen Qian, Chuanwei Zhang and Shengwang Du


https://arxiv.org/abs/2105.04602

By coherently combining advantages while largely avoiding limitations of two mainstream platforms, optical hybrid entanglement involving both discrete and continuous variables has recently garnered widespread attention and emerged as a promising idea for building heterogenous quantum networks. Different from previous results, here we propose a new scheme to remotely generate hybrid entanglement between discrete-polarization and continuous-quadrature optical qubits heralded by two-photon Bell state measurement. As a novel nonclassical light resource, we further utilize it to discuss two examples of ways – entanglement swapping and quantum teloportation – in which quantum information processing and communications could make use of this hybrid technique.


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leau - 24-12-2021 at 06:13

Quantum Ising Hamiltonian Programming in Trio, Quartet, and Sextet Qubit Systems

Minhyuk Kim, Yunheung Song, Jaewan Kim and Jaewook Ahn



PRX QUANTUM 1, 020323 (2020)
DOI: 10.1103/PRXQuantum.1.020323

Rydberg-atom quantum simulators are of keen interest because of their possibilities towards high-dimensional qubit architectures. Here we report continuous tuning of quantum Ising Hamiltonians of Rydberg atoms in three-dimensional arrangements. Various connected graphs of Rydberg atoms constructed with vertices and edges respectively representing atoms and Rydberg-blockaded atom pairs, and their eigenenergies are probed along with their geometric intermediates during structural transformations.Conformation spectra of star, complete, cyclic, and diamond graphs are probed for four interacting atoms and antiprism structures for six atoms. The energy level shifts and merges of the tested structural transformations are clearly observed with Fourier-transform spectroscopy, in good agreement with the model few-body quantum Ising Hamiltonian. This result demonstrates the possibility of continuous geometry tuning and thus programming of many-body spin-Hamiltonian systems.


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leau - 25-12-2021 at 07:48

Observing crossover between quantum speed limits

Gal Ness, Manolo R. Lam, Wolfgang Alt, Dieter Meschede, Yoav Sagi & Andrea Alberti

Sci. Adv. 7, eabj9119 (2021)
DOI: 10.1126/sciadv.abj9119

Quantum mechanics sets fundamental limits on how fast quantum states can be transformed in time. Two well-known quantum speed limits are the Mandelstam-Tamm and the Margolus-Levitin bounds, which relate the maximum speed of evolution to the system’s energy uncertainty and mean energy, respectively. Here, we test concurrently both limits in a multilevel system by following the motion of a single atom in an optical trap using fast matter wave interferometry. We find two different regimes: one where the Mandelstam-Tamm limit constrains the evolution at all times, and a second where a crossover to the Margolus-Levitin limit occurs at longer times. We take a geometric approach to quantify the deviation from the speed limit, measuring how much the quantum evolution deviates from the geodesic path in the Hilbert space of the multilevel system. Our results are important to understand the ultimate performance of quantum computing devices and related advanced quantum technologies.


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leau - 26-12-2021 at 08:05

Resonance from antiferromagnetic spin fluctuations for superconductivity in UTe 2

Chunruo Duan, R. E. Baumbach, Andrey Podlesnyak, Yuhang Deng, Camilla Moir, Alexander J. Breindel, M. Brian Maple, E. M. Nica, Qimiao Si and Pengcheng Dai

DOI: 10.1038/s41586-021-04151-5
https://arxiv.org/abs/2106.14424

Superconductivity has its universal origin in the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc. While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S=0, they can also form parallel spin-triplet Cooper pairs with S=1 and an odd parity wavefunction, analogous to the equal spin pairing state in the superfluid 3He. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for fault tolerant quantum computation. However, spin-triplet pairing is rare and has not been unambiguously identified in any solid state systems. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based heavy-fermion materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc=1.6K, has been identified as a strong candidate for chiral spin-triplet topological superconductor near a FM instability, although the system also exhibits antiferromagnetic (AF) spin fluctuations]. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation at the Brillouin zone (BZ) boundary near AF order, analogous to the resonance seen in high-Tc copper oxide, iron-based, and heavy-fermion superconductors. We find that the resonance in UTe2 occurs below Tc at an energy Er=7.9kBTc (kB is Boltzmann's constant) and at the expense of low-energy spin fluctuations. Since the resonance has only been found in spin-singlet superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations can also induce spin-triplet pairing for superconductivity.


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leau - 27-12-2021 at 08:10

Real-Time Error Correction for Quantum Computing Philip Ball Physics 14, 184 | DOI: 10.1103/Physics.14.184 Random errors incurred during computation are one of the biggest obstacles to unleashing the full power of quantum computers. Researchers have now demonstrated a technique that allows errors to be detected and corrected in real time as the computation proceeds. It also allows error correction to be conducted several times on a single quantum bit (qubit) during the calculation. Both features are needed to make the basic elements—the logical qubits—of a fully error-tolerant quantum computer that can be scaled up and used for applications beyond the specialized ones that these machines have tackled so far.

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leau - 28-12-2021 at 08:57

Universal variational quantum computation

Jacob Biamonte

PHYSICAL REVIEW A 103, L030401 (2021)
https://doi.org/10.1103/PhysRevA.103.L030401

Variational quantum algorithms dominate contemporary gate-based quantum enhanced optimization, eigen-value estimation, and machine learning. Here we establish the quantum computational universality of variational quantum computation by developing two objective functions which minimize to prepare outputs of arbitrary quantum circuits. The fleeting resource of variational quantum computation is the number of expected values which must be iteratively minimized using classical-to-quantum outer loop optimization. An efficient solution to this optimization problem is given by the quantum circuit being simulated itself. The first construction is efficient in the number of expected values for n-qubit circuits containing O(poly ln n) non-Clifford gates—the number of expected values has no dependence on Clifford gates appearing in the simulated circuit. The second approach yields O(L 2 ) expected values whereas introducing not more than O(ln L) slack qubits for a quantum circuit partitioned into L gates. Hence, the utilitarian variational quantum programming procedure—based on the classical evaluation of objective functions and iterated feedback—is, in principle, as powerful as any other model of quantum computation. This result elevates the formal standing of the variational approach whereas establishing a universal model of quantum computation.


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leau - 29-12-2021 at 07:38

Quantum Information Scrambling on a Superconducting Qutrit Processor

M. S. Blok, V. V. Ramasesh , T. Schuster, K. O’Brien, J. M. Kreikebaum, D. Dahlen, A. Morvan, B. Yoshida, N. Y. Yao and I. Siddiqi

PHYSICAL REVIEW X 11, 021010 (2021)
DOI: 10.1103/PhysRevX.11.021010


The dynamics of quantum information in strongly interacting systems, known as quantum information scrambling, has recently become a common thread in our understanding of black holes, transport in exotic non-Fermi liquids, and many-body analogs of quantum chaos. To date, verified experimental implementations of scrambling have focused on systems composed of two-level qubits. Higher-dimensional quantum systems, however, may exhibit different scrambling modalities and are predicted to saturate conjectured speed limits on the rate of quantum information scrambling. We take the first steps toward accessing such phenomena, by realizing a quantum processor based on superconducting qutrits (three-level quantum systems). We demonstrate the implementation of universal two-qutrit scrambling operations and embed them in a five-qutrit quantum teleportation protocol. Measured teleportation fidelities F avg ¼ 0.568  0.001 confirm the presence of scrambling even in the presence of experimental imperfections and decoherence. Our teleportation protocol, which connects to recent proposals for studying traversable wormholes in the laboratory, demonstrates how quantum technology that encodes information in higher-dimensional systems can exploit a larger and more connected state space to achieve the resource efficient encoding of complex quantum circuits.


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leau - 30-12-2021 at 06:19

Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer

Takahiro Kashiwazaki, Taichi Yamashima, Naoto Takanashi, Asuka Inoue, Takeshi Umeki and Akira Furusawa

Appl. Phys. Lett. 119, 251104 (2021);
https://doi.org/10.1063/5.0063118

A continuous-wave (CW) broadband high-level optical quadrature squeezer is essential for high-speed large-scale fault-tolerant quantum computing on a time-domain-multiplexed continuous-variable optical cluster state. CW THz-bandwidth squeezed light can be obtained with a waveguide optical parametric amplifier (OPA); however, the squeezing level has been insufficient for applications of fault-tolerant quantum computation because of degradation of the squeezing level due to their optical losses caused by the structural perturbation and pump-induced phenomena. Here, by using mechanical polishing processes, we fabricated a low-loss quasi-single-mode periodically poled LiNbO 3(PPLN) waveguide, which shows 7% optical propagation loss with a waveguide length of 45 mm. Using the waveguide, we assembled a low-loss fiber-pigtailed OPA module with a total insertion loss of 21%. Thanks to its directly bonded core on a LiTaO 3 substrate, the waveguide does not show pump-induced optical loss even under a condition of hundreds of milliwatts pumping. Furthermore, the quasi-single-mode structure prohibits excitation of higher-order spatial modes and enables us to obtain larger squeezing level. Even with including optical coupling loss of the modularization, we observe 6.3-dB squeezed light from the DC component up to a 6.0-THz sideband in a fully fiber-closed optical system. By excluding the losses due to imperfections of the modularization and detection, the squeezing level at the output of the PPLN waveguide is estimated to be over 10 dB. Our waveguide squeezer is a promising quantum light source for high-speed large-scale faulttolerant quantum computing.


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leau - 31-12-2021 at 07:01

Asymptotic Improvements to Quantum Circuits via Qutrits

Pranav Gokhale, Natalie C. Brown, Casey Duckering, Jonathan M. Baker, Kenneth R. Brown & Frederic T. Chong

https://www.researchgate.net/publication/333418921

Quantum computation is traditionally expressed in terms of quantum bits, or qubits. In this work, we instead consider three-level qutrits. Past work with qutrits has demonstrated only constant factor improvements, owing to the log 2 (3) binary-to-ternary compression factor. We present a novel technique using qutrits to achieve a logarithmic depth (runtime) decomposition of the Generalized Toffoli gate using no ancilla a significant improvement over linear depth for the best qubit-only equivalent. Our circuit construction also features a 70x improvement in two-qudit gate count over the qubit-only equivalent decomposition. This results in circuit cost reductions for important algorithms like quantum neurons and Grover search. We develop an open-source circuit simulator for qutrits, along with realistic near-term noise models which account for the cost of operating qutrits. Simulation results for these noise models indicate over 90% mean reliability (fidelity) for our circuit construction, versus under 30% for the qubit-only baseline. These results suggest that qutrits offer a promising path towards scaling quantum computation.


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leau - 1-1-2022 at 05:18

Prospects for Simulating a Qudit-Based Model of (1+1)d Scalar QED

Erik J. Gustafson

Phys. Rev. D 103, 114505 (2021)
DOI: 10.1103/PhysRevD.103.114505

We present a gauge invariant digitization of (1 + 1)d scalar quantum electrodynamics for an arbitrary spin truncation for qudit-based quantum computers. We provide a construction of the Trotter operator in terms of a universal qudit-gate set. The cost savings of using a qutrit based spin-1 encoding versus a qubit encoding are illustrated. We show that a simple initial state could be simulated on current qutrit based hardware using noisy simulations for two different native gate set.


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leau - 2-1-2022 at 07:18

Experimental quantum cryptography with qutrits

Simon Gröblacher, Thomas Jennewein, Alipasha Vaziri, Gregor Weihs and Anton Zeilinger

New Journal of Physics 8 (2006) 75
doi:10.1088/1367-2630/8/5/075

We produce two identical keys using, for the first time, entangled trinary quantum systems (qutrits) for quantum key distribution. The advantage of qutrits over the normally used binary quantum systems is an increased coding density and a higher security margin. The qutrits are encoded into the orbital angular momentum of photons, namely Laguerre–Gaussian modes with azimuthal index l + 1, 0 and −1, respectively. The orbital angular momentum is controlled with phase holograms. In an Ekert-type protocol the violation of a three-dimensional Bell inequality verifies the security of the generated keys.A key is obtained with a qutrit error rate of approximately 10%.


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leau - 3-1-2022 at 06:20

Quantum Cryptography Based On Bell Inequalities for Three-Dimensional System

Dagomir Kaszlikowski, Kelken Chang, D. K. L. Oi, L.C. Kwek and C.H. Oh

https://arxiv.org/abs/quant-ph/0206170

We present a crytographic protocol based upon entangled qutrit pairs. We analyse the scheme under a symmetric incoherent attack and plot the region for which the protocol is secure and compare this with the region of violations of certain Bell inequalities.


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[Edited on 3-1-2022 by leau]

leau - 4-1-2022 at 06:23

Quantum Information Scrambling on a Superconducting Qutrit Processor

M. S. Blok, V. V. Ramasesh , T. Schuster, K. O’Brien, J. M. Kreikebaum, D. Dahlen, A. Morvan, B. Yoshida, 3 N. Y. Yao and I. Siddiqi .

PHYSICAL REVIEW X 11, 021010 (2021) DOI: 10.1103/PhysRevX.11.021010

The dynamics of quantum information in strongly interacting systems, known as quantum information scrambling, has recently become a common thread in our understanding of black holes, transport in exotic non-Fermi liquids, and many-body analogs of quantum chaos. To date, verified experimental implementations of scrambling have focused on systems composed of two-level qubits. Higher-dimensional quantum systems, however, may exhibit different scrambling modalities and are predicted to saturate conjectured speed limits on the rate of quantum information scrambling. We take the first steps toward accessing such phenomena, by realizing a quantum processor based on superconducting qutrits (three-level quantum systems). We demonstrate the implementation of universal two-qutrit scrambling operations and embed them in a five-qutrit quantum teleportation protocol. Measured teleportation fidelities F avg ¼ 0.568  0.001 confirm the presence of scrambling even in the presence of experimental imperfections and decoherence. Our teleportation protocol, which connects to recent proposals for studying traversable wormholes in the laboratory, demonstrates how quantum technology that encodes information in higher-dimensional systems can exploit a larger and more connected state space to achieve the resource efficient encoding of complex quantum circuits


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leau - 5-1-2022 at 06:44

Improving wafer-scale Josephson junction resistance variation in superconducting quantum coherent circuits

J.M. Kreikebaum, K.P. O’Brien, A. Morvan, and I. Siddiqi

DOI: 10.1088/1361-6668/ab8617
Supercond. Sci. Technol. 33 06LT02 (2020)

Quantum bits, or qubits, are an example of coherent circuits envisioned for next-generation computers and detectors. A robust superconducting qubit with a coherent lifetime of O(100 µs) is the transmon: a Josephson junction functioning as a non-linear inductor shunted with a capacitor to form an anharmonic oscillator. In a complex device with many such transmons, precise control over each qubit frequency is often required, and thus variations of the junction area and tunnel barrier thickness must be sufficiently minimized to achieve optimal performance while avoiding spectral overlap between neighboring circuits. Simply transplanting our recipe optimized for single, stand-alone devices to wafer-scale (producing 64, 1x1 cm dies from a 150 mm wafer) initially resulted in global drifts in room-temperature tunneling resistance of ± 30%. Inferring a critical current I c variation from this resistance distribution, we present an optimized process developed from a systematic 38 wafer study that results in < 3.5% relative standard deviation (RSD) in critical current (≡ σ I c / hI c i) for 3000 Josephson junctions (both single-junctions and asymmetric SQUIDs) across an area of 49 cm 2 . Looking within a 1x1 cm moving window across the substrate gives an estimate of the variation characteristic of a given qubit chip. Our best process, utilizing ultrasonically assisted development, uniform ashing, and dynamic oxidation has shown σ I c / hI c i = 1.8% within 1x1 cm, on average, with a few 1x1 cm areas having σ I c / hI c i < 1.0% (equivalent to σ f / h f i < 0.5%). Such stability would drastically improve the yield of multi-junction chips with strict critical current requirements.


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leau - 6-1-2022 at 04:41

Conditional teleportation of quantum-dot spin states

Haifeng Qiao, Yadav P. Kandel, Sreenath K. Manikandan, Andrew N. Jordan Geoffrey C. Gardner, Michael J. Manfra, John M. Nichol & Saeed Fallahi

NATURE COMMUNICATIONS | (2020) 11:3022
https://doi.org/10.1038/s41467-020-16745-0

Among the different platforms for quantum information processing, individual electron spins in semiconductor quantum dots stand out for their long coherence times and potential for scalable fabrication. The past years have witnessed substantial progress in the capabilities of spin qubits. However, coupling between distant electron spins, which is required for quantum error correction, presents a challenge, and this goal remains the focus of intense research. Quantum teleportation is a canonical method to transmit qubit states, but it has not been implemented in quantum-dot spin qubits. Here, we present evidence for quantum teleportation of electron spin qubits in semiconductor quantum dots. Although we have not performed quantum state tomography to definitively assess the teleportation fidelity, our data are consistent with conditional teleportation of spin eigenstates, entanglement swapping, and gate teleportation. Such evidence for all-matter spin-state teleportation underscores the capabilities of exchange-coupled spin qubits for quantum-information transfer.


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leau - 7-1-2022 at 08:27

Can the displacemon device test objective collapse models?

Journal of Vacuum Science & Technology B 40, 012802 (2022);

https://doi.org/10.1116/6.0001485

Lydia A. Kanari-Naish, Jack Clarke, Michael R. Vanner and Edward A. Laird

Testing the limits of the applicability of quantum mechanics will deepen our understanding of the universe and may shed light on the interplay between quantum mechanics and gravity. At present there is a wide range of approaches for such macroscopic tests spanning from matter-wave interferometry of large molecules to precision measurements of heating rates in the motion of micro-scale cantilevers. The “displacemon” is a proposed electromechanical device consisting of a mechanical resonator flux-coupled to a superconducting qubit enabling generation and readout of mechanical quantum states. In the original proposal, the mechanical resonator was a carbon nanotube, containing 10 6 nucleons. Here, in order to probe quantum mechanics at a more macroscopic scale, we propose using an aluminum mechanical resonator on two larger mass scales, one inspired by the Marshall–Simon–Penrose–Bouwmeester moving-mirror proposal, and one set by the Planck mass. For such a device, we examine the experimental requirements needed to perform a more macroscopic quantum test and thus feasibly detect the decoherence effects predicted by two objective collapse models: Di osi–Penrose and continuous spontaneous localization. Our protocol for testing these two theories takes advantage of the displacemon architecture to create non-Gaussian mechanical states out of equilibrium with their environment and then analyzes the measurement statistics of a superconducting qubit. We find that with improvements to the fabrication and vibration sensitivities of these electromechanical devices, the displacemon device provides a new route to feasibly test decoherence mechanisms beyond standard quantum theory.


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leau - 8-1-2022 at 16:50

Measurement-Induced Entanglement Transitions in the Quantum Ising Chain: From Infinite to Zero Clicks

Xhek Turkeshi, Alberto Biella, Rosario Fazio, Marcello Dalmonte and Marco Schiró

Phys. Rev. B 103, 224210 (2021) DOI: 10.1103/PhysRevB.103.224210

We investigate measurement-induced phase transitions in the Quantum Ising chain coupled to a monitoring environment. We compare two different limits of the measurement problem, the stochastic quantum-state diffusion protocol corresponding to infinite small jumps per unit of time and the no-click limit, corresponding to post-selection and described by a non-Hermitian Hamiltonian. In both cases we find a remarkably similar phenomenology as the measurement strength γ is increased, namely a sharp transition from a critical phase with logarithmic scaling of the entanglement to an area-law phase, which occurs at the same value of the measurement rate in the two protocols. An effective central charge, extracted from the logarithmic scaling of the entanglement, vanishes continuously at the common transition point, although with different critical behavior possibly suggesting different universality classes for the two protocols. We interpret the central charge mismatch near the transition in terms of noise-induced disentanglement, as suggested by the entanglement statistics which displays emergent bimodality upon approaching the critical point. The non-Hermitian Hamiltonian and its associated subradiance spectral transition provide a natural framework to understand both the extended critical phase, emerging here for a model which lacks any continuous symmetry, and the entanglement transition into the area law


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leau - 9-1-2022 at 03:46

e Measurement-induced Transition in Long-range Interacting Quantum Circuits

Maxwell Block, Yimu Bao, Soonwon Choi, Ehud Altman and Norman Y. Yao

Phys. Rev. Lett. 128, 010604
https://arxiv.org/abs/2104.13372

The competition between scrambling unitary evolution and projective measurements leads to a phase transition in the dynamics of quantum entanglement. Here, we demonstrate that the nature of this transition is fundamentally altered by the presence of long-range, power-law interactions. For sufficiently weak power-laws, the measurement-induced transition is described by conformal field theory, analogous to short-range-interacting hybrid circuits. However, beyond a critical power-law, we demonstrate that long-range interactions give rise to a continuum of non-conformal universality classes, with continuously varying critical exponents. We numerically determine the phase diagram for a one-dimensional, long-range-interacting hybrid circuit model as a function of the power-law exponent and the measurement rate. Finally, by using an analytic mapping to a long-range quantum Ising model, we provide a theoretical understanding for the critical power-law.


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leau - 10-1-2022 at 07:41

Quantum State Complexity in Computationally Tractable Quantum Circuits

Jason Iaconis

PRX QUANTUM 2, 010329 (2021)

Characterizing the quantum complexity of local random quantum circuits is a very deep problem with implications to the seemingly disparate fields of quantum information theory, quantum many-body physics, and high-energy physics. While our theoretical understanding of these systems has progressed in recent years, numerical approaches for studying these models remains severely limited. In this paper, we discuss a special class of numerically tractable quantum circuits, known as quantum automaton circuits, which may be particularly well suited for this task. These are circuits that preserve the computational basis, yet can produce highly entangled output wave functions. Using ideas from quantum complexity theory, especially those concerning unitary designs, we argue that automaton wave functions have high quantum state complexity. We look at a wide variety of metrics, including measurements of the output bit-string distribution and characterization of the generalized entanglement properties of the quantum state, and find that automaton wave functions closely approximate the behavior of fully Haar random states. In addition to this, we identify the generalized out-of-time ordered 2k-point correlation functions as a particularly useful probe of complexity in automaton circuits. Using these correlators, we are able to numerically study the growth of complexity well beyond the scrambling time for very large systems. As a result, we are able to present evidence of a linear growth of design complexity in local quantum circuits, consistent with conjectures from quantum information theory.


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leau - 14-1-2022 at 10:23

Deterministic Shallow Dopant Implantation in Silicon with Detection Confidence Upper‐Bound to 99.85% by Ion‐Solid Interactions

November 2021 Advanced Materials

DOI:10.1002/adma.202103235

Alexander M. Jakob, Simon G. Robson, Vivien Schmitt, Vincent Mourik, Matthias Posselt, Daniel Spemann, Brett C. Johnson, Hannes R. Firgau, Edwin Mayes, Jeffrey C. McCallum, Andrea Morello, and David N. Jamieson

Silicon chips containing arrays of single dopant atoms could be the material of choice for both classical and quantum devices that exploit single donor spins. For example, group-V-donors implanted in isotopically purified ²⁸Si crystals are attractive for large-scale quantum computers. Useful attributes include long nuclear and electron spin lifetimes of ³¹P, hyperfine clock transitions in ²⁰⁹Bi or electrically controllable ¹²³Sb nuclear spins. Promising architectures require the ability to fabricate arrays of individual near-surface dopant atoms with high yield. Here we employ an on-chip detector electrode system with 70 eV r.m.s. noise (∼20 electrons) to demonstrate near room temperature implantation of single 14 keV ³¹P⁺ ions. The physics model for the ion-solid interaction shows an unprecedented upper-bound single ion detection confidence of 99.85±0.02% for near-surface implants. As a result, the practical controlled silicon doping yield is limited by materials engineering factors including surface gate oxides in which detected ions may stop. For a device with 6 nm gate oxide and 14 keV ³¹P⁺ implants we demonstrate a yield limit of 98.1%. Thinner gate oxides allow this limit to converge to the upper-bound. Deterministic single ion implantation can therefore be a viable materials engineering strategy for scalable dopant architectures in silicon devices.


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leau - 16-1-2022 at 09:07

Spin-orbit driven electrical manipulation of the zero-field splitting in high-spin centers in solids

Biktagirov, Timur & Gerstmann, Uwe

PHYSICAL REVIEW RESEARCH 2, 023071 (2020) DOI:10.1103/PhysRevResearch.2.023071

In recent years, spin-orbit coupling has attracted significant attention due to its promising applications in spintronic devices. In solid-state spin qubits, the spin-orbit coupling allows for the lifting of spin degeneracy in the absence of an external magnetic field. Such spin-orbit driven zero-field splitting can be directly tuned by external electric fields. Here we present a reliable theoretical framework to address this phenomenon in extended periodic systems. We unravel the microscopic origin of the zero-field splitting in light-element semiconductors and propose its implications for coherent electrical control. The reported theoretical results open up promising possibilities for a rational design and tuning of high-spin centers suitable for quantum information processing.
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leau - 18-1-2022 at 08:50

A chemical path to quantum information

Stephen von Kugelgen & Danna E Freedman

DOI: 10.1126/science.aaz4044

Science 2019 Nov 29;366(6469):1070-1071


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leau - 19-1-2022 at 11:08

Controllable freezing of the nuclear spin bath in a single-atom spin qubit

Mateusz T. Mądzik, Thaddeus D. Ladd, Fay E. Hudson, Kohei M. Itoh, Alexander M. Jakob, Brett C. Johnson, David N. Jamieson, Jeffrey C. McCallum, Andrew S. Dzurak, Arne Laucht and Andrea Morello

Sci Adv. 2020 Jul 3;6(27):eaba3442.
doi: 10.1126/sciadv.aba3442.

The quantum coherence and gate fidelity of electron spin qubits in semiconductors is often limited by noise arising from coupling to a bath of nuclear spins. Isotopic enrichment of spin-zero nuclei such as 28 Si has led to spectacular improvements of the dephasing time T 2 ∗ which, surprisingly, can extend two orders of magnitude beyond theoretical expectations. Using a single-atom 31 P qubit in enriched 28 Si, we show that the abnormally long T 2 ∗ is due to the controllable freezing of the dynamics of the residual 29 Si nuclei close to the donor. Our conclusions are supported by a nearly parameter-free modeling of the 29 Si nuclear spin dynamics, which reveals the degree of back-action provided by the electron spin as it interacts with the nuclear bath. This study clarifies the limits of ergodic assumptions in analyzing many-body spin-problems under conditions of strong, frequent measurement, and provides novel strategies for maximizing coherence and gate fidelity of spin qubits in semiconductors.


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leau - 20-1-2022 at 12:28

Quantum tomography of an entangled three-qubit state in silicon Kenta Takeda, Akito Noiri Seigo Tarucha & Takashi Nakajima Nature Nanotechnology (2021) DOI: 10.1038/s41565-021-00925-0 Quantum entanglement is a fundamental property of coherent quantum states and an essential resource for quantum computing. In large-scale quantum systems, the error accumulation requires concepts for quantum error correction. A first step toward error correction is the creation of genuinely multipartite entanglement, which has served as a performance benchmark for quantum computing platforms such as superconducting circuits, trapped ions and nitrogen-vacancy centres in diamond. Among the candidates for large-scale quantum computing devices, silicon-based spin qubits offer an outstanding nanofabrication capability for scaling-up. Recent studies demonstrated improved coherence times, high-fidelity all-electrical control, high-temperature operation and quantum entanglement of two spin qubits.Here we generated a three-qubit Greenberger–Horne–Zeilinger state using a low-disorder, fully controllable array of three spin qubits in silicon. We performed quantum state tomography and obtained a state fidelity of 88.0%. The measurements witness a genuine Greenberger–Horne–Zeilinger class quantum entanglement that cannot be separated into any biseparable state. Our results showcase the potential of silicon-based spin qubit platforms for multiqubit quantum algorithms.

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leau - 21-1-2022 at 07:00

Fidelity benchmarks for two-qubit gates in silicon

W. Huang, C. H. Yang, K. W. Chan, T. Tanttu, B. Hensen, R. C. C. Leon, M. A. Fogarty, J. C. C. Hwang, F. E. Hudson, K. M. Itoh, A. Morello, A. Laucht & A. S. Dzurak

Nature 569, 532-536 (2019)
DOI: 10.1038/s41586-019-1197-0

Universal quantum computation will require qubit technology based on a scalable platform, together with quantum error correction protocols that place strict limits on the maximum infidelities for one- and two-qubit gate operations. While a variety of qubit systems have shown high fidelities at the one-qubit level, superconductor technologies have been the only solid-state qubits manufactured via standard lithographic techniques which have demonstrated two-qubit fidelities near the fault-tolerant threshold. Silicon-based quantum dot qubits are also amenable to large-scale manufacture and can achieve high single-qubit gate fidelities (exceeding 99.9%) using isotopically enriched silicon. However, while two-qubit gates have been demonstrated in silicon, it has not yet been possible to rigorously assess their fidelities using randomized benchmarking, since this requires sequences of significant numbers of qubit operations (≳20) to be completed with non-vanishing fidelity. Here, for qubits encoded on the electron spin states of gate-defined quantum dots, we demonstrate Bell state tomography with fidelities ranging from 80% to 89%, and two-qubit randomized benchmarking with an average Clifford gate fidelity of 94.7% and average Controlled-ROT (CROT) fidelity of 98.0%. These fidelities are found to be limited by the relatively low gate times employed here compared with the decoherence times T∗2 of the qubits. Silicon qubit designs employing fast gate operations based on high Rabi frequencies, together with advanced pulsing techniques, should therefore enable significantly higher fidelities in the near future.


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leau - 23-1-2022 at 10:09

Quantum logic with spin qubits crossing the surface code threshold

https://doi.org/10.1038/s41586-021-04273-w

Xiao Xue, Maximilian Russ, Nodar Samkharadze, Brennan Undseth, Amir Sammak, Giordano Scappucci & Lieven M. K. Vandersypen

High-fidelity control of quantum bits is paramount for the reliable execution of quantum algorithms and for achieving fault tolerance—the ability to correct errors aster than they occur. The central requirement for fault tolerance is expressed in terms of an error threshold. Whereas the actual threshold depends on many details, a common target is the approximately 1% error threshold of the well-known surface code. Reaching two-qubit gate fidelities above 99% has been a long-standing major goal for semiconductor spin qubits. These qubits are promising for scaling, as they can leverage advanced semiconductor technology. Here we report a spin-based quantum processor in silicon with single-qubit and two-qubit gate fidelities, all of which are above 99.5%, extracted from gate-set tomography. The average single-qubit gate fidelities remain above 99% when including crosstalk and idling errors on the neighbouring qubit. Using this high-fidelity gate set, we execute the demanding task of calculating molecular ground-state energies using a variational quantum eigensolver algorithm. Having surpassed the 99% barrier for the two-qubit gate fidelity, semiconductor qubits are well positioned on the path to fault tolerance and to possible applications in the era of noisy intermediate-scale quantum devices.


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[Edited on 24-1-2022 by leau]

leau - 24-1-2022 at 08:09

Fast universal quantum control above the fault-tolerance threshold in silicon

Akito Noiri, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Amir Sammak, Giordano Scappucci, and Seigo Tarucha

Nature 2022 Jan;601(7893):338-342.
doi: 10.1038/s41586-021-04182-y.

Fault-tolerant quantum computers which can solve hard problems rely on quantum error correction. One of the most promising error correction codes is the surface code, which requires universal gate fidelities exceeding the error correction threshold of 99 per cent. Among many qubit platforms, only superconducting circuits, trapped ions, and nitrogen-vacancy centers in diamond have delivered those requirements. Electron spin qubits in silicon are particularly promising for a large-scale quantum computer due to their nanofabrication capability, but the two-qubit gate fidelity has been limited to 98 per cent due to the slow operation. Here we demonstrate a two-qubit gate fidelity of 99.5 per cent, along with single-qubit gate fidelities of 99.8 per cent, in silicon spin qubits by fast electrical control using a micromagnet-induced gradient field and a tunable two-qubit coupling. We identify the condition of qubit rotation speed and coupling strength where we robustly achieve high-fidelity gates. We realize Deutsch-Jozsa and Grover search algorithms with high success rates using our universal gate set. Our results demonstrate the universal gate fidelity beyond the fault-tolerance threshold and pave the way for scalable silicon quantum computers.


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leau - 25-1-2022 at 11:12

The Potential Impact of Quantum Computers on Society

Ronald de Wolf

Ethics and Information Technology, 19(4):271-276, 2017

https://arxiv.org/abs/1712.05380

This paper considers the potential impact that the nascent technology of quantum computing may have on society. It focuses on three areas: cryptography, optimization, and simulation of quantum systems. We will also discuss some ethical aspects of these developments, and ways to mitigate the risks.


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[Edited on 25-1-2022 by leau]

[Edited on 25-1-2022 by leau]

leau - 26-1-2022 at 11:50

Probing Topological Spin Liquids on a Programmable Quantum Simulator

G. Semeghini, H. Levine, A. Keesling, S. Ebadi, T. T. Wang, D. Bluvstein, R. Verresen, H. Pichler, M. Kalinowski, R. Samajdar, A. Omran, S. Sachdev, A. Vishwanath, M. Greiner, V. Vuletić & M. D. Lukin

https://arxiv.org/abs/2104.04119
DOI:10.1126/science.abi8794

Quantum spin liquids, exotic phases of matter with topological order, have been a major focus of explorations in physical science for the past several decades. Such phases feature long-range quantum entanglement that can potentially be exploited to realize robust quantum computation.we use a 219-atom programmable quantum simulator to probe quantum spin liquid states. In our approach, arrays of atoms are placed on the links of a kagome lattice and evolution under Rydberg blockade creates frustrated quantum states with no local order. The onset of a quantum spin liquid phase of the paradigmatic toric code type is detected by evaluating topological string operators that provide direct signatures of topological order and quantum correlations. Its properties are further revealed by using an atom array with nontrivial topology, representing a first step towards topological encoding. Our observations enable the controlled experimental exploration of topological quantum matter and protected quantum information processing.


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leau - 27-1-2022 at 09:46

The Search for the Quantum Spin Liquid in Kagome Antiferromagnets

J.-J. Wen, Y. S. Lee

CHIN. PHYS. LETT. Vol. 36, No. 5 (2019) 050101
DOI: 10.1088/0256-307X/36/5/050101

We systematically study the low-temperature specific heats for the two-dimensional kagome antiferromagnet, Cu3Zn(OH)6FBr. The specific heat exhibits a T1.7 dependence at low temperatures and a shoulder-like feature above it. We construct a microscopic lattice model of Z2 quantum spin liquid and perform large-scale quantum Monte Carlo simulations to show that the above behaviors come from the contributions from gapped anyons and magnetic impurities. Surprisingly, we find the entropy associated with the shoulder decreases quickly with grain size d, although the system is paramagnetic to the lowest temperature. While this can be simply explained by a core-shell picture in that the contribution from the interior state disappears near the surface, the 5.9-nm shell width precludes any trivial explanations. Such a large length scale signifies the coherence length of the nonlocality of the quantum entangled excitations in quantum spin liquid candidate, similar to Pippard's coherence length in superconductors. Our approach therefore offers a new experimental probe of the intangible quantum state of matter with topological order.


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leau - 28-1-2022 at 09:53

Quantum spin liquids in a square lattice subject to an Abelian flux and its experimental observation in cold atoms or photonic systems

Fadi Sun and Jinwu Ye

https://arxiv.org/abs/2005.04695

We report that a possible Z 2 quantum spin liquid (QSL) can be observed in a new class of frustrated system: spinor bosons subject to a π flux in a square lattice. We construct a new class of Ginsburg-Landau (GL) type of effective action to classify possible quantum or topological phases at any coupling strengths. It can be used to reproduce the frustrated SF with the 4 sublattice 90 ◦coplanar spin structure plus its excitations in the weak coupling limit and the FM Mott plus its excitations in the strong coupling limit achieved in our previous work. It also establishes deep and intrinsic connections between the GL effective action and the order from quantum disorder (OFQD) phenomena in the weak coupling limit. Most importantly, it predicts two possible new phases at intermediate couplings: a FM SF phase or a frustrated magnetic Mott phase. We argue that the latter one is more likely and melts into a Z 2 quantum spin liquid (QSL) phase. If the heating issue can be under a reasonable control at intermediate couplings U/t ∼ 1, the topological order of thhe Z 2 QSL maybe uniquely probed by the current cold atom or photonic experimental techniques


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leau - 29-1-2022 at 05:33

Preparing for Quantum-Safe Cryptography
An NCSC whitepaper about mitigating the threat to cryptography from development in Quantum Computing.

https://www.ncsc.gov.uk/whitepaper/preparing-for-quantum-saf...


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leau - 30-1-2022 at 06:32

Quantum phases of Rydberg atoms on a kagome lattice

Rhine Samajdar, Wen Wei Ho, Hannes Pichler , Mikhail D. Lukin, and Subir Sachdev

Proc Natl Acad Sci. 2021 Jan 26;118(4):e2015785118.
doi: 10.1073/pnas.2015785118.

We analyze the zero-temperature phases of an array of neutral atoms on the kagome lattice, interacting via laser excitation to atomic Rydberg states. Density-matrix renormalization group calculations reveal the presence of a wide variety of complex solid phases with broken lattice symmetries. In addition, we identify a regime with dense Rydberg excitations that has a large entanglement entropy and no local order parameter associated with lattice symmetries. From a mapping to the triangular lattice quantum dimer model, and theories of quantum phase transitions out of the proximate solid phases, we argue that this regime could contain one or more phases with topological order. Our results provide the foundation for theoretical and experimental explorations of crystalline and liquid states using programmable quantum simulators based on Rydberg atom arrays.


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leau - 31-1-2022 at 09:49

Evidence for a Z 2 topological ordered quantum spin liquid in a kagome-lattice antiferromagnet

Yuan Wei, Zili Feng, Wiebke Lohstroh, D. H. Yu, Clarina dela Cruz, Wei Yi, Z. F. Ding, J. Zhang, Cheng Tan, Lei Shu, Yan-Cheng Wang, Han-Qing Wu, Jianlin Luo, Jia-Wei Mei, Zi Yang Meng, Youguo Shi and Shiliang Li

https://arxiv.org/abs/1710.02991v1

A quantum spin liquid with a Z 2 topological order has long been thought to be important for the application of quantum computing and may be related to high-temperature superconductivity. While a two-dimensional kagome antiferromagnet may host such a state, strong experimental evidences are still lacking]. Here we show that the spin excitations from the kagome planes in magnetically ordered Cu 4 (OD) 6 FBr and non-magnetically ordered Cu 3 Zn(OD) 6 FBr are similarly gapped although the content of inter-kagome-layer Cu 2+ ions changes dramatically. This suthat the spin triplet gap and continuum of the intrinsic kagome antiferromagnet are robust against the interlayer magnetic impurities. Our results show that the ground state of Cu 3 Zn(OD) 6 FBr is a gapped quantum spin liquid with Z 2 topological order.


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leau - 1-2-2022 at 11:18

Scheme for the generation of hybrid entanglement between time-bin and wavelike encodings

Élie Gouzien, Floriane Brunel, Sébastien Tanzilli, and Virginia D’Auria

DOI : 10.1103/PhysRevA.102.012603
https://arxiv.org/abs/2002.04450
HAL Id : hal-02474883, version 2

We propose a scheme for the generation of hybrid states entangling a single-photon time-bin qubit with a coherent-state qubit encoded on phases. Compared to other reported solutions, time-bin encoding makes hybrid entanglement particularly well adapted to applications involving long-distance propagation in optical fibers. This makes our proposal a promising resource for future out-of-the-laboratory quantum communication. In this perspective, we analyze our scheme by taking into account realistic experimental resources and discuss the impact of their imperfections on the quality of the obtained hybrid state


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leau - 2-2-2022 at 08:37

A hybrid quantum repeater for qudits

Marcel Bergmann and Peter van Loock

https://arxiv.org/abs/1708.09322 Phys. Rev. A 99, 032349

We present a "hybrid quantum repeater" protocol for the long-distance distribution of atomic entangled states beyond qubits. In our scheme, imperfect noisy entangled pairs of two qudits, i.e., two discrete-variable d-level systems, each of, in principle, arbitrary dimension d, are initially shared between the intermediate stations of the channel. This is achieved via local, sufficiently strong light-matter interactions, involving optical coherent states and their transmission after these interactions, and optical measurements on the transmitted field modes, especially (but not restricted to) efficient continuous-variable homodyne detections ("hybrid" here refers to the simultaneous exploitation of discrete and continuous variable degrees of freedom for the local processing and storage of entangled states as well as their non-local distribution, respectively). For qutrits we quantify the light-matter entanglement that can be effectively shared through an elementary lossy channel, and for a repeater spacing of up to 10 km we show that the realistic (lossy) qutrit entanglement is even larger than any ideal (loss-free) qubit entanglement. After including qudit entanglement purification and swapping procedures, we calculate the long-distance entangled-pair distribution rates and the final entangled-state fidelities for total communication distances of up to 1280 km. With three rounds of purification, entangled qudit pairs of near-unit fidelity can be distributed over 1280 km at rates of the order of, in principle, 100 Hz.


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leau - 3-2-2022 at 08:24

Superconductivity in an extreme strange metal

D. H. Nguyen, A. Sidorenko, M. Taupin, G. Knebel, G. Lapertot, E. Schuberth & S. Paschen

https://doi.org/10.1038/s41467-021-24670-z

Some of the highest-transition-temperature superconductors across various materials classes exhibit linear-in-temperature ‘strange metal’ or ‘Planckian’ electrical resistivities in their normal state. It is thus believed by many that this behavior holds the key to unlock the secrets of high-temperature superconductivity. However, these materials typically display complex phase diagrams governed by various competing energy scales, making an unambiguous identification of the physics at play difficult. Here we use electrical resistivity measurements into the micro-Kelvin regime to discover superconductivity condensing out of an extreme strange metal state—with linear resistivity over 3.5 orders of magnitude in temperature. We propose that the Cooper pairing is mediated by the modes associated with a recently evidenced dynamical charge localization–delocalization transition, a mechanism that may well be pertinent also in other strange metal superconductors.


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leau - 4-2-2022 at 07:09

Multicomponent superconducting order parameter in UTe 2

I. M. Hayes, D. S. Wei, T. Metz, J. Zhang, Y. S. Eo, S. Ran, S. R. Saha , J. Collini, N. P. Butch, D. F. Agterberg, A. Kapitulnik & J. Paglione

Cite as: I. M. Hayes et al., Science
DOI: 10.1126/science.abb0272 (2021).

An unconventional superconducting state was recently discovered in UTe 2 , where spin-triplet superconductivity emerges from the paramagnetic normal state of a heavy fermion material. The coexistence of magnetic fluctuations and superconductivity, together with the crystal structure of this material, suggest that a unique set of symmetries, magnetic properties, and topology underlie the superconducting state. Here, we report observations of a non-zero polar Kerr effect and of two transitions in the specific heat upon entering the superconducting state, which together suggest that the superconductivity in UTe 2 is characterized by a two-component order parameter that breaks time reversal symmetry. These data place constraints on the symmetries of the order parameter and inform the discussion on the presence of topological superconductivity in UTe 2 .


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leau - 5-2-2022 at 09:54

Spatially inhomogeneous superconductivity in UTe 2

S. M. Thomas, C. Stevens, F. B. Santos, S. S. Fender, E. D. Bauer, F. Ronning, J. D. Thompson, A. Huxley, and P. F. S. Rosa

July 2021Science 373(6556):eabb0272
DOI:10.1126/science.abb0272

Newly-discovered superconductor UTe 2 is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe 2 at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T c ). Notably, samples with higher T c only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe 2 .


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leau - 6-2-2022 at 05:30

Universal Scaling Law for the Condensation Energy, U, Across a Broad Range of Superconductor Classes

J. S. Kim, G. N. Tam, and G. R. Stewart

DOI:10.1103/PhysRevB.92.224509
https://arxiv.org/abs/1512.06144

One of the goals in understanding any new class of superconductors is to search for commonalities with other known superconductors. The present work investigates the superconducting condensation energy, U, in the iron based superconductors (IBS), and compares their U with a broad range of other distinct classes of superconductor, including conventional BCS elements and compounds and the unconventional heavy Fermion, Sr 2 RuO 4 , Li 0.1 ZrNCl, (BEDT-TTF) 2 Cu(NCS) 2 and optimally doped cuprate superconductors. Surprisingly, both the magnitude and T c dependence (U @ T c3.4±0.2 ) of U are – contrary to the previously observed behavior of the specific heat discontinuity at T c , C, – quite similar in the IBS and BCS materials for T c >1.4 K. In contrast, the heavy Fermion superconductors’ U vs T c are strongly (up to a factor of 100) enhanced above the IBS/BCS while the cuprate superconductors’ U are strongly (factor of 8) reduced. However, scaling of U with the specific heat (or C) brings all the superconductors investigated onto one universal dependence upon T c . This apparent universal scaling U T c2 for all superconductor classes investigated, both weak and strong coupled and both conventional and unconventional, links together extremely disparate behaviors over almost seven orders of magnitude for U and almost three orders of magnitude for T c . Since U has not yet been explicitly calculated beyond the weak coupling limit, the present results can help direct theoretical efforts into the medium and strong coupling regimes.


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leau - 7-2-2022 at 04:12

Core-Level Photoelectron Spectroscopy Study of UTe 2

Shin-ichi Fujimori, Ikuto Kawasaki, Yukiharu Takeda, Hiroshi Yamagami, Ai Nakamura, Yoshiya Homma and Dai Aoki

Journal of the Physical Society of Japan 90, 015002 (2021)
DOI: 10.7566/JPSJ.90.015002

The valence state of UTe 2 was studied by core-level photoelectron spectroscopy. The main peak position of the U 4 f core-level spectrum of UTe 2 coincides with that of UB 2 , which is an itinerant compound with a nearly 5 f 3 configuration. However, the main peak of UTe 2 is broader than that of UB 2 , and satellite structures are observed in the higher binding energy side of the main peak, which are characteristics of mixed-valence uranium compounds. These results suggest that the U 5 f state in UTe 2 is in a mixed valence state with a dominant contribution from the itinerant 5 f 3 configuration.


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leau - 8-2-2022 at 13:56

Theory of spin-polarized superconductors –an analogue of superfluid 3 He A-phase

Kazushige Machida

https://arxiv.org/abs/2101.08963
Phys. Rev. B 104, 014514

It is shown theoretically that ferromagnetic superconductors, UGe 2 , URhGe, and UCoGe can be described in terms of the A-phase like triplet pairing similar to superfluid 3 He in a unified way, including peculiar reentrant, S-shape, or L-shape H c2 curves. The associated double transition inevitable between the A 1 and A 2 -phases in the H-T plane is predicted, both of which are characterized by non-unitary state with broken time reversal symmetry and the half-gap. UTe 2 , which has been discovered quite recently to be a spin-polarized superconductor, is analyzed successively in the same view point, pointing out that the expected A 1 -A 2 transition is indeed emerging experimentally. Thus the four heavy Fermion compounds all together are entitled to be topologically rich solid state materials worth further investigating together with superfluid 3 He A-phase.


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leau - 9-2-2022 at 08:06

Spin-Orbit Coupling Induced Degeneracy in the Anisotropic Unconventional Superconductor UTe2

Alexander B. Shick, Warren E. Pickett

Phys. Rev. B 100, 134502 (2019)
DOI:10.1103/PhysRevB.100.134502

The orthorhombic uranium dichalcogenide UTe2 displays superconductivity below 1.7 K, with the anomalous feature of retaining 50% of normal state (ungapped) carriers, according to heat capacity data from two groups. Incoherent transport that crosses over from above 50 K toward a low temperature, Kondo lattice Fermi liquid regime indicates strong magnetic fluctuations and the need to include correlation effects in theoretical modeling. We report density functional theory plus Hubbard U (DFT+U) results for UTe2 to provide a platform for modeling its unusual behavior, focusing on ferromagnetic (FM, time reversal breaking) long range correlations along the a^ axis as established by magnetization measurements and confirmed by our calculations. States near the Fermi level are dominated by the j=52 configuration, with the jz=±12 sectors being effectively degenerate and half-filled. Unlike the small-gap insulating nonmagnetic electronic spectrum, the FM Fermi surfaces are large (strongly metallic) and display low dimensional features, reminiscent of the FM superconductor UGe2.


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leau - 10-2-2022 at 06:31

Spin Susceptibility of the Topological Superconductor UPt 3 from Polarized Neutron Diffraction

Phys. Rev. B 96, 041111 (2017)
doi:10.1103/PhysRevB.96.041111

W. J. Gannon, W. P. Halperin, M. R. Eskildsen, Pengcheng Dai, U. B. Hansen, K. Lefmann & A. Stunault

Experiment and theory indicate that UPt 3 is a topological superconductor in an odd-parity state, based in part from temperature independence of the NMR Knight shift. However, quasiparticle spin-flip scattering near a surface, where the Knight shift is measured, might be responsible. We use polarized neutron scattering to measure the bulk susceptibility with H||c, finding consistency with the Knight shift but inconsistent with theory for this field orientation. We infer that neither spin susceptibility nor Knight shift are a reliable indication of odd-parity.


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leau - 11-2-2022 at 10:38

Magnetic Properties under Pressure in Novel Spin-Triplet Superconductor UTe 2

Journal of the Physical Society of Japan 90, 073703 (2021)
https://doi.org/10.7566/JPSJ.90.073703 Dexin Li, Ai Nakamura, Fuminori Honda, Yoshiki

J. Sato, Yoshiya Homma, Yusei Shimizu, Jun Ishizuka, Youichi Yanase, Georg Knebel, Jacques Flouquet, and Dai Aoki We report the magnetic susceptibility and the magnetization under pressures up to 1.7 GPa above the critical pressure, P c ∼ 1.5 GPa, for H ∥ a, b, and c-axes in the novel spin triplet superconductor UTe 2 . The anisotropic magnetic susceptibility at low pressure with the easy magnetization a-axis changes to the quasi-isotropic behavior at high pressure, revealing a rapid suppression of the susceptibility for a-axis, and a gradual increase of the susceptibility for the b-axis. At 1.7 GPa above P c , magnetic anomalies are detected at T MO ∼ 3 K and T WMO ∼ 10 K. The former anomaly corresponds to long-range magnetic order, most likely antiferromagnetism, while the latter shows a broad anomaly, which is probably due to the development of short range order. The unusual decrease and increase of the susceptibility below T WMO for H ∥ a and b-axes, respectively, indicate the complex magnetic properties at low temperatures above P c . This is related to the interplay between multiple fluctuations dominated by antiferromagnetism, ferromagnetism, valence and Fermi surface instabilities.


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leau - 12-2-2022 at 10:29

Feedback of superconductivity on the magnetic excitation spectrum of UTe 2

http://arxiv.org/abs/2107.13914v1
DOI:10.7566/JPSJ.90.113706

Stéphane Raymond, William Knafo, Georg Knebel, Koji Kaneko, Jean-Pascal Brison, Jacques Flouquet, Dai Aoki & Gérard Lapertot

We investigate the spin dynamics in the superconducting phase of UTe 2 by triple-axis inelastic neutron scattering on a single crystal sample. At the wave-vector k 1 =(0, 0.57, 0), where the normal state antiferromagnetic correlations are peaked, a modification of the excitationspectrum is evidenced, on crossing the superconducting transition, with a reduction of the relaxation rate together with the development of an inelastic peak at Ω ≈ 1 meV. The low dimensional nature and the the a-axis polarization of the fluctuations, that characterise the normal state, are essentially maintained below T sc . The high ratio Ω/k B T sc ≈ 7.2 contrasts with the most common behaviour in heavy fermion superconductors.

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leau - 13-2-2022 at 13:05

Enabling Dataflow Optimization for Quantum Programs

David Ittah, Thomas Häner, Vadym Kliuchnikov, Torsten Hoefler

https://arxiv.org/abs/2101.11030

We propose an IR for quantum computing that directly exposes quantum and classical data dependencies for the purpose of optimization. The Quantum Intermediate Representation for Optimization (QIRO) consists of two dialects, one input dialect and one that is specifically tailored to enable quantum-classical co-optimization. While the first employs a perhaps more intuitive memory-semantics (quantum operations act as side-effects), the latter uses value-semantics (operations consume and produce states). Crucially, this encodes the dataflow directly in the IR, allowing for a host of optimizations that leverage dataflow analysis. We discuss how to map existing quantum programming languages to the input dialect and how to lower the resulting IR to the optimization dialect. We present a prototype implementation based on MLIR that includes several quantum-specific optimization passes. Our benchmarks show that significant improvements in resource requirements are possible even through static optimization. In contrast to circuit optimization at run time, this is achieved while incurring only a small constant overhead in compilation time, making this a compelling approach for quantum program optimization at application scale.


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leau - 15-2-2022 at 10:59

A systematic mapping on quantum software development in the context of software engineering

Paulo Eduardo Zanni Jr & Valter Vieira de Camargo

https://arxiv.org/abs/2106.00926

Quantum Computing is a new paradigm that enables several advances which are impossible using classical technology. With the rise of quantum computers, the software is also invited to change so that it can better fit this new computation way. However, although a lot of research is being conducted in the quantum computing field, it is still scarce studies about the differences of the software and software engineering in this new context. Therefore, this article presents a systematic mapping study to present a wide review on the particularities and characteristics of software that are developed for quantum computers. A total of 24 papers were selected using digital libraries with the objective of answering three research questions elaborated in the conduct of this research.


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leau - 17-2-2022 at 08:54

An attack to quantum systems through RF radiation tracking

Kadir Durak , Naser Jam

https://arxiv.org/pdf/2004.14445.pdf

A newfound security breach in the physical nature of single photon detectors that are generally used in quantum key distribution is explained, we found that the bit contents of a quantum key transmission system can be intercepted from far away by exploiting the ultrawideband electromagnetic signals radiated from hi-voltage avalanche effect of single photon detectors. It means that in fact any Geiger mode avalanche photodiode that is used inside single photon detectors systematically acts like a downconverter that converts the optical-wavelength photons to radio-wavelength photons that can be intercepted by an antenna as side channel attack. Our experiment showed that the radiated waveforms captured by the antenna can be used as a fingerprint. These finger prints were fed to a deep learning neural network as training data, and after training the neural network was able to clone the bit content of quantum transmission.


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leau - 18-2-2022 at 09:18

Cryptographic Attack Possibilities over RSA Algorithm through Classical and Quantum Computation

Kapil Kumar Soni Akhtar Rasool

International Conference on Smart Systems and Inventive Technology (ICSSIT 2018) IEEE Xplore Part Number: CFP18P17-ART; ISBN:978-1-5386-5873-4

Cryptographic attack possibilities have several parameters and one of the possibilities is to attack over the cryptographic algorithm. Large integer factorization is still a challenging problem since the emergence of mathematics and computer science. Benchmark cryptographic protocol, the RSA Algorithm requires factorization of large integers.Classical computation does not have any polynomial time algorithm that can factor any arbitrary large integer. The remarkable but not efficient, classical algorithms for integer factorization are Trial Division, General Number Field Sieve and Quadratic Sieve. The influence of Shor’s algorithm assures to get the efficient solution of such factorization problem in polynomial time and challenges the security parameters of the existing cryptosystem, but algorithm implementation limits to be executed on a quantum computer.


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leau - 19-2-2022 at 11:07

An Efficient Quantum Computing technique for cracking RSA using Shor’s Algorithm

Vaishali Bhatia & K.R. Ramkumar

2020 IEEE 5th International Conference on Computing Communication and Automation (ICCCA) Galgotias University, Greater Noida, UP, India. Oct 30-31, 2020
DOI: 10.1109/ICCCA49541.2020.9250806

Quantum Computing is a prominent word in this era as it allows computation to be performed in no time. The motive of using Quantum Computing (QC) is that even exponentially large number of problems can be solved using it which was earlier difficult with the classical computing. Conventional methods are based on usage of bits which consist of 0’s and 1’s while QC works with qubits. The main issue is that conventional computing has issue of storage as well as computation even when parallel computation is performed on it. Concept of quantum parallelism allows the computation to be performed in exponentially very low time as compared to conventional method. This paper will discuss about Quantum Computing Algorithms and how Shor’s algorithm is able to break RSA algorithms is discussed. Entanglement and superposition of qubits helps fast computation. The demonstration of the applicability has been evaluated based on computation time, storage capacity, accuracy, confidentiality, efficiency, integrity, and availability. Among various algorithms Shor’s technique is able to break various encryption algorithm with more supremacy as compared to conventional computing methods. In nutshell, paper will discuss about various QC algorithms and will illustrate how shor’s algorithm is able to crack RSA.


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leau - 20-2-2022 at 06:43

Is Your Quantum Program Bug-Free?

Andriy Miranskyy, Lei Zhang and Javad Doliskani

https://doi.org/10.1145/3377816.3381731
https://arxiv.org/abs/2001.10870

Quantum computers are becoming more mainstream. As more programmers are starting to look at writing quantum programs, they face an inevitable task of debugging their code. How should the programs for quantum computers be debugged? In this paper, we discuss existing debugging tactics, used in developing programs for classic computers, and show which ones can be readily adopted. We also highlight quantum-computer-specific debugging issues and list novel techniques that are needed to address these issues. The practitioners can readily apply some of these tactics to their process of writing quantum programs, while researchers can learn about opportunities for future work.


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leau - 21-2-2022 at 08:24

Are Heavy Fermion Strange Metals Planckian?

Mathieu Taupin and Silke Paschen

https://doi.org/10.3390/cryst12020251
Crystals 12, no. 2: 251.

Strange metal behavior refers to a linear temperature dependence of the electrical resistivity at temperatures below the Mott-Ioffe-Regel limit. It is seen in numerous strongly correlated electron systems, from the heavy fermion compounds, via transition metal oxides and iron pnictides, to magic angle twisted bi-layer graphene, frequently in connection with unconventional or “high temperature” superconductivity. To achieve a unified understanding of these phenomena across the different materials classes is a central open problem in condensed matter physics. Tests whether the linear-in-temperature law might be dictated by Planckian dissipation—scattering with the rate ∼ k B T/h̄, are receiving considerable attention. Here we assess the situation for strange metal heavy fermion compounds. They allow to probe the regime of extreme correlation strength, with effective mass or Fermi velocity renormalizations in excess of three orders of magnitude. Adopting the same procedure as done in previous studies, i.e., assuming a simple Drude conductivity with the above scattering rate, we find that for these strongly renormalized quasiparticles, scattering is much weaker than Planckian, implying that the linear temperature dependence should be due to other effects. We discuss implications of this finding and point to directions for further work.


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leau - 22-2-2022 at 10:33

Connecting heterogeneous quantum networks by hybrid entanglement swapping

Giovanni Guccione, Tom Darras, Hanna Le Jeannic, Varun B. Verma, Sae Woo Nam, Adrien Cavaillès and Julien Laurat

https://doi.org/10.1126/sciadv.aba4508
https://arxiv.org/abs/2003.11041v2

Recent advances in quantum technologies are rapidly stimulating the building of quantum networks. With the parallel development of multiple physical platforms and different types of encodings, a challenge for present and future networks is to uphold a heterogeneous structure for full functionality and therefore support modular systems that are not necessarily compatible with one another. Central to this endeavor is the capability to distribute and interconnect optical entangled states relying on different discrete and continuous quantum variables. Here, we report an entanglement swapping protocol connecting such entangled states. We generate single-photon entanglement and hybrid entanglement between particle- and wave-like optical qubits and then demonstrate the heralded creation of hybrid entanglement at a distance by using a specific Bell-state measurement. This ability opens up the prospect of connecting heterogeneous nodes of a network, with the promise of increased integration and novel functionalities.


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leau - 23-2-2022 at 08:49

Remote preparation of continuous-variable qubits using loss-tolerant hybrid entanglement of light

H. Le Jeannic, A. Cavaillès, J. Raskop, K. Huang, and J. Laurat

https://doi.org/10.1364/OPTICA.5.001012
https://arxiv.org/abs/1809.10700

Transferring quantum information between distant nodes of a network is a key capability. This transfer can be realized via remote state preparation where two parties share entanglement and the sender has full knowledge of the state to be communicated. Here we demonstrate such a process between heterogeneous nodes functioning with different information encodings, i.e., particle-like discrete-variable optical qubits and wave-like continuous-variable ones. Using hybrid entanglement of light as a shared resource, we prepare arbitrary coherent-state superpositions controlled by measurements on the distant discrete-encoded node. The remotely prepared states are fully characterized by quantum state tomography and negative Wigner functions are obtained. This work demonstrates a novel capability to bridge discrete- and continuous-variable platforms.


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leau - 25-2-2022 at 10:51

Conference 10082: Solid State Lasers XXVI: Technology and Devices

SPIE Photonics West 2017

Part of Proceedings of SPIE Vol. 10082 Solid State Lasers XXVI: Technology and Devices


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leau - 26-2-2022 at 11:12

UTe 2 : a nearly insulating half-filled j =25f 3 heavy fermion metal

Alexander B. Shick, Shin-ichi Fujimori, Warren E. Pickett

Phys. Rev. B 103, 125136 (2021)
https://doi.org/10.48550/arXiv.2103.11410

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [DFT+U(ED)] of the Anderson impurity term with Coulomb repulsion U in the open 14-orbital 5f shell is applied to UTe 2 . The small gap for U =0, evidence of the half-filled j = 52 subshell of 5f 3 uranium, is converted for U =3 eV to a flat band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. Two means of identification from the Green’s function give a mass enhancement of the order of 12 for already heavy (flat) bands, consistent with the common heavy fermion characterization of UTe 2 . The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by DFT+U(ED) to differ by a factor of seven, indicating a strong site distinction, while the anisotropy factor η = 0.18 is similar for all three sites. The calculated uranium moment < M 2 > 1/2 of 3.5µ B is roughly consistent with the published experimental Curie-Weiss values of 2.8µ B and 3.3µ B (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund’s rule values for an f 3 ion. The U =3 eV spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong 5f character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of UTe 2 is that of a half-filled j = 52 subshell with two half-filled m j = ± 2 1 orbitals forming a narrow gap by hybridization, then driven to a conducting state by configuration mixing (spin-charge fluctuations). UTe 2 displays similarities to UPt 3 with its 5f dominated Fermi surfaces rather than a strongly localized Kondo lattice system.


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[Edited on 26-2-2022 by leau]

leau - 27-2-2022 at 10:24

Quantum restricted Boltzmann machine universal for quantum computation

Yusen Wu, Chunyan Wei, Sujuan Qin, Qiaoyan Wen, Fei Gao

https://arxiv.org/abs/2005.11970
https://doi.org/10.48550/arXiv.2005.1197

The challenge posed by the many-body problem in quantum physics originates from the difficulty of describing the nontrivial correlations encoded in the many-body wave functions with high complexity. Quantum neural network provides a powerful tool to represent the large-scale wave function, which has aroused widespread concern in the quantum superiority era. A significant open problem is what exactly the representational power boundary of the single-layer quantum neural network is. In this paper, we design a 2-local Hamiltonian and then give a kind of Quantum Restricted Boltzmann Machine (QRBM, i.e. single-layer quantum neural network) based on it. The proposed QRBM has the following two salient features. (1) It is proved universal for implementing quantum computation tasks. (2) It can be efficiently implemented on the Noisy Intermediate-Scale Quantum (NISQ) devices. We successfully utilize the proposed QRBM to compute the wave functions for the notable cases of physical interest including the ground state as well as the Gibbs state (thermal state) of molecules on the superconducting quantum chip. The experimental results illustrate the proposed QRBM can compute the above wave functions with an acceptable error.


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leau - 28-2-2022 at 11:03

Quantum Humanities: A First Use Case for Quantum-ML in Media Science

Johanna Barzen & Frank Leymann

https://doi.org/10.1007/s42354-019-0243-2 .

Quantum computers are becoming real. Therefore, it is promising to use their potentials in different applications areas, which includes research in the humanities.Due to an increasing amount of data that needs to be processed in the digital humanities the use of quantum computers can contribute to this research area. To give an impression on how beneficial such involvement of quantum computers can be when analyzing data from the humanities, a use case from the media science is presented. Therefore, both the theoretical basis and the tooling support for analyzing the data from our digital humanities project MUSE is described. This includes a data analysis pipeline, containing e.g. various approaches for data preparation, feature engineering, clustering, and classification where several steps can be realized classically, but also supported by quantum computers.


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leau - 1-3-2022 at 11:59

Feature Learning with Gaussian Restricted Boltzmann Machine for Robust Speech Recognition

Xin Zheng, Zhiyong Wu, Helen Meng, Weifeng Li, Lianhong Cai

https://arxiv.org/abs/1309.6176

In this paper, we first present a new variant of Gaussian restricted Boltzmann machine (GRBM) called multivariate Gaussian restricted Boltzmann machine (MGRBM), with its definition and learning algorithm. Then we propose using a learned GRBM or MGRBM to extract better features for robust speech recognition. Our experiments on Aurora2 show that both GRBM-extracted and MGRBM-extracted feature performs much better than Mel-frequency cepstral coefficient (MFCC) with either HMM-GMM or hybrid HMM-deep neural network (DNN) acoustic model, and MGRBM-extracted feature is slightly better.


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leau - 2-3-2022 at 10:35

An Algorithm of Quantum Restricted Boltzmann Machine Network Based on Quantum Gates and Its Application

Peilin Zhang, Sheng Li and Yu Zhou

https://doi.org/10.1155/2015/756969

We present an algorithm of quantum restricted Boltzmann machine network based on quantum gates. The algorithm is used to initialize the procedure that adjusts the qubit and weights. After adjusting, the network forms an unsupervised generative model that gives better classification performance than other discriminative models. In addition, we show how the algorithm can be constructed with quantum circuit for quantum computer.


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leau - 4-3-2022 at 12:21

Multiorbital spin-triplet pairing and spin resonance in the heavy-fermion superconductor UTe 2

Lei Che, Haoyu Hu, Christopher Lane, Emilian M. Nica, Jian-Xin Zhu, and Qimiao

https://arxiv.org/abs/2112.14750
https://doi.org/10.48550/arXiv.2112.14750

The heavy-fermion system UTe 2 is a candidate for spin-triplet superconductivity, which is of considerable interest to quantum engineering. Among the outstanding issues is the nature of the pairing state. A recent surprising discovery is the observation of a resonance in the spin excitation spectrum at an antiferromagneticwavevector [C. Duan et al., Nature 600, 636 (2021)], which stands in apparent contrast to the ferromagnetic nature of the interactions expected in this system. We show how the puzzle can be resolved by a multiorbitalspin-triplet pairing constructed from local degrees of freedom. Because it does not commute with the kinetic part of the Hamiltonian, the pairing contains both intra- and inter- band terms in the band basis. We demonstrate that the intraband pairing component naturally yields a spin resonance at the antiferromagnetic wavevector. Our work illustrates how orbital degrees of freedom can enrich the nature and properties of spin-triplet superconductivity of strongly-correlated quantum materials.


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leau - 5-3-2022 at 13:53

Implementation of a Walsh-Hadamard Gate in a Superconducting Qutrit

M A Yurtalan, J Shi, M Kononenko, A Lupascu & S Ashhab

DOI: 10.1103/PhysRevLett.125.180504
Phys Rev Lett. 2020 Oct 30;125(18):180504
https://arxiv.org/abs/2003.04879

We have implemented a Walsh-Hadamard gate, which performs a quantum Fourier transform, in a superconducting qutrit. The qutrit is encoded in the lowest three energy levels of a capacitively shunted flux device, operated at the optimal flux-symmetry point. We use an efficient decomposition of the Walsh-Hadamard gate into two unitaries, generated by off-diagonal and diagonal Hamiltonians, respectively. The gate implementation utilizes simultaneous driving of all three transitions between the three pairs of energy levels of the qutrit, one of which is implemented with a two-photon process. The gate has a duration of 35 ns and an average fidelity over a representative set of states, including preparation and tomography errors, of 99.2%, characterized with quantum-state tomography. Compensation of ac-Stark and Bloch-Siegert shifts is essential for reaching high gate fidelities.


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leau - 6-3-2022 at 12:26

Generalized Optical Signal Processing Based on Multi-Operator Metasurfaces Synthesized by Susceptibility Tensors

Ali Momeni, Hamid Rajabalipanah, Ali Abdolali, Karim Achouri

https://doi.org/10.48550/arXiv.1811.02618
https://arxiv.org/abs/1811.02618

This paper theoretically proposes a multichannel functional metasurface computer characterized by Generalized Sheet Transition Conditions (GSTCs) and surface susceptibility tensors. The study explores a polarization- and angle-multiplexed metasurfaces enabling multiple and independent parallel analog spatial computations when illuminated by differently polarized incident beams from different directions. The proposed synthesis overcomes substantial restrictions imposed by previous designs such as large architectures arising from the need of additional subblocks, slow responses, and most importantly, supporting only the even reflection symmetry operations for normal incidences, working for a certain incident angle or polarization, and executing only single mathematical operation. The versatility of the design is demonstrated in a way that an ultra-compact, integrable and planar metasurface-assisted platform can execute a variety of optical signal processing operations such as spatial differentiation and integration. It is demonstrated that a metasurface featuring non-reciprocal property can be thought of as a new paradigm to break the even symmetry of reflection and perform both even- and odd-symmetry mathematical operations at normal incidences. Numerical simulations also illustrate different aspects of multichannel edge detection scheme through projecting multiple images on the metasurface from different directions. Such appealing findings not only circumvent the major potential drawbacks of previous designs but also may offer an efficient, easy-to-fabricate, and flexible approach in wave-based signal processing, edge detection, image contrast enhancement, hidden object detection, and equation solving without any Fourier lens.


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leau - 7-3-2022 at 11:34

Biology begins to tangle with quantum computing

Vivien Marx

Nat Methods 18, 715–719 (2021).

https://doi.org/10.1038/s41592-021-01199-z


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leau - 8-3-2022 at 07:49

Quantum circuit architecture search on a superconducting processor

Kehuan Linghu, Yang Qian, Ruixia Wang, Meng-Jun Hu, Zhiyuan Li, Xuegang Li, Huikai Xu, Jingning Zhang, Teng Ma, Peng Zhao, Dong E. Liu, Min-Hsiu Hsieh, Xingyao Wu, Yuxuan Du, Dacheng Tao, Yirong Jin & Haifeng Yu

https://arxiv.org/abs/2201.00934
https://doi.org/10.48550/arXiv.2201.00934

Variational quantum algorithms (VQAs) have shown strong evidences to gain provable computational advantages for diverse fields such as finance, machine learning, and chemistry. However, the heuristic ansatz exploited in modern VQAs is incapable of balancing the tradeoff between expressivity and trainability, which may lead to the degraded performance when executed on the noisy intermediate-scale quantum (NISQ) machines. To address this issue, here we demonstrate the first proof-of-principle experiment of applying an efficient automatic ansatz design technique, i.e., quantum architecture search (QAS), to enhance VQAs on an 8-qubit superconducting quantum processor. In particular, we apply QAS to tailor the hardware-efficient ansatz towards classification tasks. Compared with the heuristic ansatze, the ansatz designed by QAS improves test accuracy from 31% to 98%. We further explain this superior performance by visualizing the loss landscape and analyzing effective parameters of all ansatze. Our work provides concrete guidance for developing variable ansatze to tackle various large-scale quantum learning problems with advantages.


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leau - 9-3-2022 at 09:41

Reciprocal space study of Heisenberg exchange interactions in ferromagnetic metals

I. V. Kashin, A. Gerasimov, V. V. Mazurenko

https://arxiv.org/abs/2112.13016
https://doi.org/10.48550/arXiv.2112.13016

The modern quantum theory of magnetism in solids is getting commonly derived using Green's functions formalism. The popularity draws itself from remarkable opportunities to capture the microscopic landscape of exchange interactions, starting from a tight-binding representation of the electronic structure. Indeed, the conventional method of infinitesimal spin rotations, considered in terms of local force theorem, opens vast prospects of investigations regarding the magnetic environment, as well as pairwise atomic couplings. However, this theoretical concept practically does not devoid of intrinsic inconsistencies. In particular, naturally expected correspondence between single and pairwise infinitesimal spin rotations is being numerically revealed to diverge. In this work, we elaborate this question on the model example and canonical case of bcc iron. Our analytical derivations discovered the principal preference of on-site magnetic precursors if the compositions of individual atomic interactions are in focus. The problem of extremely slow or even absent spatial convergence while considering metallic compounds was solved by suggesting the original technique, based on reciprocal space framework. Using fundamental Fourier transform-inspired interconnection between suggested technique and traditional spatial representation, we shed light on symmetry breaking in bcc Fe on the level of orbitally decomposed total exchange surrounding.


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leau - 10-3-2022 at 10:31

PREPARING FOR Q-DAY

Davide Castelvecchi

Nature, Vol 602, 10 February 2022 pp 198-201

The quantum-computer revolution could give hackers superpowers. New encryption algorithms will keep them at bay.


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leau - 11-3-2022 at 09:19

Quantum Computing: How to Address the National Security Risk

Dr. Arthur Herman & Idalia Friedson

2018 Hudson Institute


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