By Dr. Chris Mansell

Shown below are summaries of a few interesting research papers in quantum computing and communications that have been published over the past month.

Hardware

Title: Entanglement across separate silicon dies in a modular superconducting qubit device
Organizations: Rigetti Computing; NASA Ames Research Center (QuAIL); USRA Research Institute for Advanced Computer Science (RIACS)
Assembling future large-scale quantum computers out of smaller, specialized modules promises to simplify a number of formidable science and engineering challenges. One of the primary challenges in developing a modular architecture is in engineering high fidelity, low-latency quantum interconnects between modules. Here we demonstrate a modular solid state architecture with deterministic inter-module coupling between four physically separate, interchangeable superconducting qubit integrated circuits, achieving two-qubit gate fidelities as high as 99.1 ± 0.5% and 98.3 ± 0.3% for iSWAP and CZ entangling gates, respectively.
https://www.nature.com/articles/s41534-021-00484-1

Title: Quantum Speed-up in Collisional Battery Charging
Organizations: Université de Genève; Universität Siegen
Reducing the charging time for batteries is a significant engineering challenge for electric vehicles and personal electronics but using quantum effects to improve battery technology has only been researched for less than a decade. In this article where the rate at which energy can be transferred to a single quantum battery is investigated, the main finding is that quantum coherence and interference can lead to faster charging than is possible for classical batteries. A concrete physical implementation was not discussed but since collective, many-body processes are not required for this theoretical proposal, demonstrating the results experimentally might be comparatively straightforward.
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.100601

Title: Molecular Machines for Quantum Error Correction
Organization: Pontifícia Universidade Católica
Biological reactions are remarkably precise with some molecular machines such as RNA polymerase having an error rate of less than one in a million. Quantum technologies have not come close to this despite operating in the most pristine environments that we can engineer. By taking inspiration from biology, the author considers a quantum mechanical robot, or qubot, that could autonomously assist a quantum computer with its error correction duties. Since the interactions between superconductors and a variety of other quantum computing platforms have already been studied in the context of hybrid quantum devices, the author envisions the qubot as a small number of superconducting loops, which would act to protect the quantum information from disorder. 
https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.030336

Title: Light-induced valleytronics in pristine graphene
Organizations: Indian Institute of Technology; Max-Born Institut

After the first detailed experimental characterisation of graphene in 2004, this one-atom thick sheet of carbon with superlative properties has been regarded as a wonder material. Within computing, graphene has the potential to operate at room temperature with a clock rate millions of times faster than modern computer cores. To date, this has only worked if the graphene was put under strain, had impurities added or been combined with other two-dimensional materials. In this theoretical paper, the authors conceive of a way to do this with pristine graphene, opening a long but exciting path towards engineering graphene qubits.
https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-3-422&id=449329

Title (1): Hexagonal Boron Nitride (hBN) as a Low-loss Dielectric for Superconducting Quantum Circuits and Qubits
Organizations (1): Massachusetts Institute of Technology; National Institute for Materials Science, Japan
https://arxiv.org/abs/2109.00015
Title (2): Miniaturizing transmon qubits using van der Waals materials
Organizations (2): Columbia University; Raytheon BBN Technologies; National Institute for Materials Science, Japan
https://arxiv.org/abs/2109.02824
Soon after the flurry of research into graphene and other atomically thin materials began, the idea of stacking these monolayers – in almost arbitrary combinations – took hold. In two concurrent papers, these so-called van der Waals heterostructures were used to replace certain parts of a superconducting qubit making the entire object orders of magnitude smaller. Given that these were the first ever qubits of their kind, they displayed impressive coherence times. There are also a huge number of avenues to explore, from incorporating more advance fabrication methods to exploring new device geometries.

Title: Quantum Design for Advanced Qubits
Organizations: University of Science and Technology of China; Shanghai Research Center for Quantum Sciences
How did we go from punch-card based computers that took up entire rooms to today’s high tech consumer electronics? More than one positive feedback loop lead to this runaway success but there is no doubt that using computers to design better computers played a vital role. A recent paper points out that since quantum computers should excel at simulating quantum mechanical systems, including the ones out of which they are made, another virtuous circle of reinforcing improvements could be at hand. The authors employed a superconducting quantum computer to design an upgraded superconducting qubit, which they then built and tested. While this was merely a proof of the principle, the future looks bright for when the feedback can truly kick in.
https://arxiv.org/abs/2109.00994

Title: Engineering high-coherence superconducting qubits
Organizations: University of California, Berkeley; Lawrence Berkeley National Laboratory
Both academia and industry are presently engaged in intense global research and development efforts on superconducting qubits. To help keep up with the dizzying rate of findings, this review paper provides a thoughtful grounding in the foundations of the field, describes the state-of-the-art results and outlines some possible directions for future research.
https://www.nature.com/articles/s41578-021-00370-4

Software

Title: Fundamental limitations of quantum error mitigation
Organizations: Nanyang Technological University; NTT Corporation; Nagoya University; National University of Singapore
The goal of quantum error correction is to achieve fault tolerant quantum computation while the more modest aim of error mitigation techniques is to reduce the impact of noise on the results obtained from noisy quantum hardware. The downside of these techniques is that the quantum circuits have to be run and re-run many times. By considering a broad class of mitigation strategies, the authors of this article find a general lower bound on the number of runs. Applied to variational algorithms subject to local depolarizing noise, the bound tells us that the number of repetitions must scale exponentially with the circuit depth. In a related work, Patrick Coles and coauthors also derive this exponential scaling. The hope for better performing error mitigation protocols now lies outside this collection of techniques.
https://arxiv.org/abs/2109.04457

Title: Cross-Verification of Independent Quantum Devices
Organizations: University of Vienna; VitreaLab GmbH; Entropica Labs; National University of Singapore; Singapore University of Technology and Design; Universität Innsbruck; University of Oxford; Österreichische Akademie der Wissenschaften; Alpine Quantum Technologies GmbH; Horizon Quantum Computing
In general, today’s quantum computers are too small and error-prone to outperform conventional computers. However, there are certain, slightly contrived tasks where classical computers cannot keep up with their quantum counterparts. Given the worldwide efforts to improve quantum technologies, there may come a time when quantum devices also excel at extremely important calculations. There is a concern that we won’t be able to double-check their results using a trustworthy, ordinary supercomputer. In this paper, the researchers have invented a procedure to compare the outputs of different quantum computers against one another and confirm that their final answer is approximately correct.
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.031049

Title: Practical Quantum Error Correction with the XZZX Code and Kerr-Cat Qubits
Organizations: Kyoto University; Japan Science and Technology Agency; University of Sydney; Yale University
The goal of fault tolerant quantum computation requires complementary advances in both software and hardware. Recent algorithmic insights have doubled the ability of the XZZX code to suppress errors in quantum computers, while Kerr-cat qubits have shown improving physical performance when implemented in a superconducting platform that goes by the acronym SNAIL. What unites these apparently disparate developments is their relationship to noise. The XZZX code works best when one type of noise is more prominent than any others and this is exactly the situation in which the Kerr-cat qubits find themselves. The simulations reported in this paper indicate that this combination of code and hardware could lead to fault-tolerant quantum computing in the foreseeable future
https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.030345

Title: Quantum Coding with Low-Depth Random Circuits
Organizations: NIST / University of Maryland; Princeton University; Harvard University; Massachusetts Institute of Technology; The University of Chicago; AWS Center for Quantum Computing
Quantum error correction is absolutely necessary for the future of quantum computing but it is currently considered extremely demanding in terms of the number of additional qubits and operations it requires. There is a trade-off because although it is possible to correct errors without too many extra qubits, this is at the expense of the runtime and while alternative methods allow for a short runtime, they require many more qubits. By considering random ways of encoding information into a quantum computer, the authors of this paper considerably improved this situation, bringing the prospect of practical error correction close to realisation.
https://journals.aps.org/prx/abstract/10.1103/PhysRevX.11.031066

Title: Quantum algorithms for group convolution, cross-correlation, and equivariant transformations
Organizations: Massachusetts Institute of Technology; Scuola Normale Superiore
Symmetry has played a foundational and pivotal role in the history of both mathematics and physics. Under the banner of “geometric deep learning,” it is now starting to prominently impact the field of machine learning. In this topical work, researchers – including giants of quantum mechanics like Seth Loyd – describe novel quantum algorithms that generalise some earlier results and provide exponential speedups over comparable classical algorithms. In doing so, they also construct a theoretical framework that could spur the design of other quantum procedures that use symmetry to process data more efficiently.
https://arxiv.org/abs/2109.11330

Title: Quantum Machine Learning for Finance
Organization: JPMorgan Chase Bank
Finance may be one of the first industries to benefit from quantum technologies. In this comprehensive review paper, the authors consider a wide range of quantum machine learning techniques, from classification and clustering to regression and reinforcement learning. They evaluate how they could be applied to various financial use cases, from accounting and algorithmic trading to risk assessment and stock selection.
https://arxiv.org/abs/2109.04298

Title: Low depth amplitude estimation on a trapped ion quantum computer
Organizations: Goldman, Sachs & Co., Stanford University, IonQ, QC Ware, and IRIF CNRS
Amplitude estimation is a fundamental quantum algorithmic primitive that enables quantum computers to achieve quadratic speedups for a large class of statistical estimation problems, including Monte Carlo methods. The main drawback from the perspective of near term hardware implementations is that the amplitude estimation algorithm requires very deep quantum circuits. Recent works have succeeded in somewhat reducing the necessary resources for such algorithms, by trading off some of the speedup for lower depth circuits, but high quality qubits are still needed for demonstrating such algorithms. Here, we report the results of an experimental demonstration of amplitude estimation on a state-of the-art trapped ion quantum computer.
https://arxiv.org/abs/2109.09685

September 29, 2021