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Author: Subject: Claimed quantum computer breakthough
leau
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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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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[*] posted on 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]
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[*] posted on 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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[*] posted on 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]

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[*] posted on 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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[*] posted on 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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