JournalTOCs

Quantum Science and Technology
Number of Followers: 8

Hybrid journal (It can contain Open Access articles)
ISSN (Online) 2058-9565
Published by IOP

[45 journals]

Magic state distillation and cost analysis in fault-tolerant universal
quantum computation
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  Authors: Yiting Liu; Zhi Ma, Lan Luo, Chao Du, Yangyang Fei, Hong Wang, Qianheng Duan Jing Yang
  First page: 043001
  Abstract: Magic states have been widely studied in recent years as resource states that help quantum computers achieve fault-tolerant universal quantum computing. The fault-tolerant quantum computing requires fault-tolerant implementation of a set of universal logical gates. Stabilizer code, as a commonly used error correcting code with good properties, can apply the Clifford gates transversally which is fault tolerant. But only Clifford gates cannot realize universal computation. Magic states are introduced to construct non-Clifford gates that combine with Clifford operations to achieve universal quantum computing. Since the preparation of quantum states is inevitably accompanied by noise, preparing the magic state with high fidelity and low overhead is the crucial problem to realizing universal quantum computation. In this paper, we survey the related literature in the past 20 years and introduce the common types of magic states, the protocols to obtain high-fidelity magic states, and overhead analysis for these protocols. Finally, we discuss the future directions of this field.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-02T23:00:00Z
  DOI: 10.1088/2058-9565/ace6ca
  Issue No: Vol. 8, No. 4 (2023)
Microgravity facilities for cold atom experiments
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  Authors: Matthias Raudonis; Albert Roura, Matthias Meister, Christoph Lotz, Ludger Overmeyer, Sven Herrmann, Andreas Gierse, Claus Lämmerzahl, Nicholas P Bigelow, Maike Lachmann, Baptist Piest, Naceur Gaaloul, Ernst M Rasel, Christian Schubert, Waldemar Herr, Christian Deppner, Holger Ahlers, Wolfgang Ertmer, Jason R Williams, Nathan Lundblad Lisa Wörner
  First page: 044001
  Abstract: Microgravity platforms enable cold atom research beyond experiments in typical laboratories by removing restrictions due to the gravitational acceleration or compensation techniques. While research in space allows for undisturbed experimentation, technological readiness, availability and accessibility present challenges for experimental operation. In this work we focus on the main capabilities and unique features of ground-based microgravity facilities for cold atom research. A selection of current and future scientific opportunities and their high demands on the microgravity environment are presented, and some relevant ground-based facilities are discussed and compared. Specifically, we point out the applicable free fall times, repetition rates, stability and payload capabilities, as well as programmatic and operational aspects of these facilities. These are contrasted with the requirements of various cold atom experiments. Besides being an accelerator for technology development, ground-based microgravity facilities allow fundamental and applied research with the additional benefit of enabling hands-on access to the experiment for modifications and adjustments.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-09T23:00:00Z
  DOI: 10.1088/2058-9565/ace1a3
  Issue No: Vol. 8, No. 4 (2023)
Quantum optimisation for continuous multivariable functions by a
structured search
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  Authors: Edric Matwiejew; Jason Pye Jingbo B Wang
  First page: 045013
  Abstract: Solving optimisation problems is a promising near-term application of quantum computers. Quantum variational algorithms (QVAs) leverage quantum superposition and entanglement to optimise over exponentially large solution spaces using an alternating sequence of classically tunable unitaries. However, prior work has primarily addressed discrete optimisation problems. In addition, these algorithms have been designed generally under the assumption of an unstructured solution space, which constrains their speedup to the theoretical limits for the unstructured Grover’s quantum search algorithm. In this paper, we show that QVAs can efficiently optimise continuous multivariable functions by exploiting general structural properties of a discretised continuous solution space with a convergence that exceeds the limits of an unstructured quantum search. We present the quantum multivariable optimisation algorithm and demonstrate its advantage over pre-existing methods, particularly when optimising high-dimensional and oscillatory functions.
  Citation: Quantum Science and Technology
  PubDate: 2023-07-30T23:00:00Z
  DOI: 10.1088/2058-9565/ace6cc
  Issue No: Vol. 8, No. 4 (2023)
Tunable capacitor for superconducting qubits using an InAs/InGaAs
heterostructure
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  Authors: Nicholas Materise; Matthieu C Dartiailh, William M Strickland, Javad Shabani Eliot Kapit
  First page: 045014
  Abstract: Adoption of fast, parametric coupling elements has improved the performance of superconducting qubits, enabling recent demonstrations of quantum advantage in randomized sampling problems. The development of low loss, high contrast couplers is critical for scaling up these systems. We present a blueprint for a gate-tunable coupler realized with a two-dimensional electron gas in an InAs/InGaAs heterostructure. Rigorous numerical simulations of the semiconductor and high frequency electromagnetic behavior of the coupler and microwave circuitry yield an on/off ratio of more than one order of magnitude. We give an estimate of the dielectric-limited loss from the inclusion of the coupler in a two qubit system, with coupler coherences ranging from a few to tens of microseconds.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-03T23:00:00Z
  DOI: 10.1088/2058-9565/aceb18
  Issue No: Vol. 8, No. 4 (2023)
Quantum chip design optimization and automation in superconducting coupler
architecture
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  Authors: Fei-Yu Li; Li-Jing Jin
  First page: 045015
  Abstract: Superconducting coupler architecture demonstrates great potential for scalable and high-performance quantum processors, yet how to design efficiently and automatically ‘Qubit–Coupler–Qubit (QCQ)’ of high performance from the layout perspective remains obscure. In this work, this issue is studied for the first time resulting in three key findings. Firstly, we acquire the crucial zero-coupling condition that is only dependent on the geometric design of the layout. Secondly, the upper bound of the qubit–qubit effective coupling is found as which surprisingly depends only on the artificially pre-decided quantities instead of specific layouts. Thirdly, we propose an optimal layout design procedure to reach the very upper bound, leading to efficient and high-performance layout design. The effectiveness of the procedure has been demonstrated scrupulously using electromagnetic simulation experiments. As a stirring application, we report a state-of-the-art 3202 um long-range and scalable QCQ layout that is especially crucial to quantum error correction. Our work provides practical guides to optimize the performance of the existing coupler architecture, find out novel layouts, and further advance the progress of quantum chip design automation.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-03T23:00:00Z
  DOI: 10.1088/2058-9565/ace8b6
  Issue No: Vol. 8, No. 4 (2023)
Faster variational quantum algorithms with quantum kernel-based surrogate
models
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  Authors: Alistair W R Smith; A J Paige M S Kim
  First page: 045016
  Abstract: We present a new optimization strategy for small-to-intermediate scale variational quantum algorithms (VQAs) on noisy near-term quantum processors which uses a Gaussian process surrogate model equipped with a classically-evaluated quantum kernel. VQAs are typically optimized using gradient-based approaches however these are difficult to implement on current noisy devices, requiring large numbers of objective function evaluations. Our approach shifts this computational burden onto the classical optimizer component of these hybrid algorithms, greatly reducing the number of quantum circuit evaluations required from the quantum processor. We focus on the variational quantum eigensolver (VQE) algorithm and demonstrate numerically that these surrogate models are particularly well suited to the algorithm’s objective function. Next, we apply these models to both noiseless and noisy VQE simulations and show that they exhibit better performance than widely-used classical kernels in terms of final accuracy and convergence speed. Compared to the typically-used stochastic gradient-descent approach to VQAs, our quantum kernel-based approach is found to consistently achieve significantly higher accuracy while requiring less than an order of magnitude fewer quantum circuit executions. We analyze the performance of the quantum kernel-based models in terms of the kernels’ induced feature spaces and explicitly construct their feature maps. Finally, we describe a scheme for approximating the best-performing quantum kernel using a classically-efficient tensor network representation of its input state and so provide a pathway for scaling this strategy to larger systems.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-09T23:00:00Z
  DOI: 10.1088/2058-9565/aceb87
  Issue No: Vol. 8, No. 4 (2023)
Quantum two-way time transfer over a hybrid free-space and fiber link
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  Authors: Xiao Xiang; Bingke Shi, Runai Quan, Yuting Liu, Zhiguang Xia, Huibo Hong, Tao Liu, Jincai Wu, Jia Qiang, Jianjun Jia, Shougang Zhang Ruifang Dong
  First page: 045017
  Abstract: As the superiority of quantum two-way time transfer (Q-TWTT) has been proved convincingly over fiber links, its implementation on free-space links becomes an urgent need for remote time transfer expanding to the transcontinental distance. In this paper, the first Q-TWTT experimental demonstration over a hybrid link of 2 km-long turbulent free-space and 7 km-long field fiber is reported. Despite the significant loss of ∼30 dB and atmospheric turbulence, reliable time transfer performance lasting for overnights has been realized with time stability in terms of time deviation far below 1 picosecond. This achievement shows the good feasibility of quantum-enhanced time transfer in the space-ground integrated optical links and nicely certifies the capability of Q-TWTT in comparing and synchronizing the state-of-the-art space microwave atomic clocks.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-20T23:00:00Z
  DOI: 10.1088/2058-9565/acedc9
  Issue No: Vol. 8, No. 4 (2023)
Diagnostics of quantum-gate coherences deteriorated by unitary errors via
end-point-measurement statistics
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  Authors: Ilaria Gianani; Alessio Belenchia, Stefano Gherardini, Vincenzo Berardi, Marco Barbieri Mauro Paternostro
  First page: 045018
  Abstract: Quantum coherence is a central ingredient in quantum physics with several theoretical and technological ramifications. We consider a figure of merit encoding the information on how the coherence generated on average by a quantum gate is affected by unitary errors (coherent noise sources) in the form of rotation-angle and rotation-axis errors. We provide numerical evidences that such information is well captured by the statistics of local energy measurements on the output states of the gate. These findings are then corroborated by experimental data taken in a quantum optics setting.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-21T23:00:00Z
  DOI: 10.1088/2058-9565/acedca
  Issue No: Vol. 8, No. 4 (2023)
Resource frugal optimizer for quantum machine learning
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  Authors: Charles Moussa; Max Hunter Gordon, Michal Baczyk, M Cerezo, Lukasz Cincio Patrick J Coles
  First page: 045019
  Abstract: Quantum-enhanced data science, also known as quantum machine learning (QML), is of growing interest as an application of near-term quantum computers. Variational QML algorithms have the potential to solve practical problems on real hardware, particularly when involving quantum data. However, training these algorithms can be challenging and calls for tailored optimization procedures. Specifically, QML applications can require a large shot-count overhead due to the large datasets involved. In this work, we advocate for simultaneous random sampling over both the dataset as well as the measurement operators that define the loss function. We consider a highly general loss function that encompasses many QML applications, and we show how to construct an unbiased estimator of its gradient. This allows us to propose a shot-frugal gradient descent optimizer called Refoqus (REsource Frugal Optimizer for QUantum Stochastic gradient descent). Our numerics indicate that Refoqus can save several orders of magnitude in shot cost, even relative to optimizers that sample over measurement operators alone.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-22T23:00:00Z
  DOI: 10.1088/2058-9565/acef55
  Issue No: Vol. 8, No. 4 (2023)
Post-selection in noisy Gaussian boson sampling: part is better than whole
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  Authors: Tian-Yu Yang; Yi-Xin Shen, Zhou-Kai Cao Xiang-Bin Wang
  First page: 045020
  Abstract: Gaussian boson sampling (GBS) is originally proposed to show quantum advantage with quantum linear optical elements. Recently, several experimental breakthroughs based on GBS pointing to quantum computing supremacy have been presented. However, due to technical limitations, the outcomes of GBS devices are influenced severely by photon loss. Here, we present a practical method to reduce the negative effect caused by photon loss. We first show with explicit formulas that a GBS process can be mapped to another GBS processes. Based on this result, we propose a post-selection method which discards low-quality data according to our criterion to improve the performance of the final computational results, say part is better than whole. As an example, we show that the post-selection method can turn a GBS experiment that would otherwise fail in a ‘non-classicality test’ into one that can pass that test. Besides improving the robustness of computation results of current GBS devices, this post-selection method may also benefit the further development of GBS-based quantum algorithms.
  Citation: Quantum Science and Technology
  PubDate: 2023-08-28T23:00:00Z
  DOI: 10.1088/2058-9565/acf06c
  Issue No: Vol. 8, No. 4 (2023)