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: Demonstration of Density Matrix Exponentiation Using a Superconducting Quantum Processor
Organizations: Massachusetts Institute of Technology; Chalmers University of Technology; Duke University
In 2014, a theoretical paper showed that multiple physical copies of a quantum state could be used to construct a unitary based on that state. Known as density matrix exponentiation, the original idea was to use it to perform a quantum version of principal component analysis. In the subsequent years, several other applications have been conceived but none have been realised in an experiment. This new work uses superconducting transmon qubits to perform circuits that are over 70 operations deep and show the algorithm working in practice. The lessons that can be learnt from implementing such an important procedure on a NISQ device are definitely valuable but the full power of the approach will only be seen once we are in the fault-tolerant era.
Link: https://journals.aps.org/prx/abstract/10.1103/PhysRevX.12.011005

Title: Ultrastable Free-Space Laser Links for a Global Network of Optical Atomic Clocks
Organization: The University of Western Australia
A global network of highly stable free-space laser links would be extremely useful for a variety of applications, not least quantum key distribution. Unfortunately, atmospheric turbulence causes both phase noise and amplitude noise. New research has demonstrated a link with a fractional stability of 6.1 x 10^{-21}, which is not only 100 times more stable than previous links but also beats the stability of modern atomic clocks. The achievement relied on using a continuous wave laser, a tilting mirror and tight temperature controls in addition to tried and tested noise-reduction techniques. Ultimately, this could increase the transmission distances and the key rates of quantum communications schemes.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.020801

Title: Towards practical quantum computers: transmon qubit with a lifetime approaching 0.5 milliseconds
Organization: Beijing Academy of Quantum Information Sciences
Superconducting qubits have shorter coherence times than those found in other platforms such as ions, atoms or nitrogen-vacancy centers. A simple and popular design for superconducting qubits is the transmon. It can be built out of a range of possible materials but the best-performing options seem to be aluminium or niobium. In this new research, a transmon qubit made from tantalum was found to systematically outperform both these materials. Importantly, the transmon was manufactured with a dry etching process that is suitable for industrial scale fabrication. A certain amount of optimisation lead to a coherence time of 0.5 milliseconds and it appears that more research and development from similar research groups around the world could extend that to milliseconds or more.
Link: https://www.nature.com/articles/s41534-021-00510-2

Title: Quantum Neuronal Sensing of Quantum Many-Body States on a 61-Qubit Programmable Superconducting Processor
Organizations: University of Science and Technology of China; Shanghai Research Center for Quantum Sciences; SOKENDAI; NTT Basic Research Laboratories and Research Center for Theoretical Quantum Physics; National Institute of Informatics; Okinawa Institute of Science and Technology Graduate University
The authors of this impressive experimental paper successfully train a quantum computer to determine whether a many-body quantum state is ergodic or localised. Their quantum circuit begins with the generation of a highly entangled state that they would like to classify. It proceeds with the implementation of a quantum neural network and concludes with the measurement of a single qubit, the result of which indicates what type of state was originally created. They used classical computation to check their classifications and experimented with ever larger quantum states. They employed 64 superconducting qubits arranged in a square grid, although they excluded three that were not sufficiently functional. The classification accuracy for the largest, 61-qubit states was only slightly lower than for the smaller-scale 16-qubit states, highlighting the scalability of their approach. 
Link: https://scirate.com/arxiv/2201.05957

Title (1): Quantum logic with spin qubits crossing the surface code threshold
Organizations (1): Delft University of Technology; Netherlands Organisation for Applied Scientific Research
Link (1): https://www.nature.com/articles/s41586-021-04273-w

Title (2): Precision tomography of a three-qubit donor quantum processor in silicon
Organizations (2): Delft University of Technology; University of Copenhagen; Université Grenoble Alpes; UNSW Sydney; University of Technology Sydney; Ain Shams University; Sandia National Laboratories; Keio University; University of Melbourne
Link (2): https://www.nature.com/articles/s41586-021-04292-7

Title (3): Fast universal quantum gate above the fault-tolerance threshold in silicon
Organizations (3): RIKEN; Delft University of Technology; Netherlands Organisation for Applied Scientific Research; 
Link (3): https://www.nature.com/articles/s41586-021-04182-y

Summary of (1), (2) and (3): Previously, there had been two scalable quantum platforms that have reached the fault tolerance threshold of the surface code: ion traps and superconductors. As described in three new Nature papers, silicon now joins them. Two of the experiments were on silicon quantum dots and the third used phosporous impurities in the silicon. By using improved materials, new approaches to the logic operations and more careful calibration methods, the different research groups demonstrated impressive single- and two-qubit gates before showing them off by running some quantum algorithms. The future looks bright because of silicon’s compatibility with advanced semiconductor manufacturing technology. The next milestone will be to improve the qubit readout fidelity.

Software

Title: Quantum computational advantage via high-dimensional Gaussian boson sampling
Organizations: NIST / University of Maryland; Caltech; Xanadu Quantum Technologies; University of Toronto; École Polytechnique de Montréal; Freie Universität Berlin; Helmholtz-Zentrum Berlin für Materialien und Energie; The University of Chicago; Universität Ulm
It may be possible to demonstrate a quantum advantage by implementing the Gaussian boson sampling (GBS) protocol with a photonic quantum computer. However, most of today’s experimental platforms are not programmable and have prohibitively high rates of photon loss. Furthermore, the underlying theory doesn’t have quite the same level of rigour as other quantum advantage schemes. The authors of this paper propose a way to overcome these issues by modifying the protocol in a way that allows optical delay lines and fast, programmable optical switches to be used. This also enables asymptotic, complexity-theoretic arguments to be made. By their estimates, optical technology already has the capability to perform their new protocol in regimes that a classical supercomputer would find completely intractable.
Link: https://www.science.org/doi/10.1126/sciadv.abi7894

Title: Quadratic Speed-Up for Simulating Gaussian Boson Sampling
Organizations: Xanadu; École Polytechnique de Montréal; University of Bristol; Imperial College London; University of Edinburgh
The previous paper was an example of how improvements to quantum protocols can make it seem like quantum processors are about to overtake their classical counterparts. However, it is important to remember that normal computers can put their foot on the accelerator as well. For example, this paper describes a new classical algorithm for simulating standard Gaussian boson sampling that is quadratically faster than the previous state of the art. This still leaves quantum devices with exponentially better scaling but it is notable because it shows that the race is not a forgone conclusion – developments are happening too frequently for that. The immediate ramifications for ambitious experimental groups are that their set-ups for conventional Gaussian boson sampling now only complete with single workstations rather than large supercomputing clusters.
Link: https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.010306

Title: QNet: A Scalable and Noise-Resilient Quantum Neural Network Architecture for Noisy Intermediate-Scale Quantum Computers
Organization: Pennsylvania State University
Quantum neural networks (QNNs) can solve supervised machine learning tasks but not at scale due to the limitations of NISQ devices. The authors of this article suggest that if you have access to several quantum devices capable of implementing QNNs, then you could give each of them a fraction of the input vector. After applying a classical nonlinear activation function, you could shuffle their outputs and feed them back into the QNNs. The authors classically simulated how well such an approach would fare and found that on average, over an extensive set of classification datasets, the accuracy would be 43% higher than for a basic QNN. The simulations included the most prominent types of noise that would occur in actual quantum hardware with the exception of cross-talk errors. Follow-up work will surely investigate if the accuracy and noise-resilience can be maintained when actually implemented rather than simulated.
Link: https://www.frontiersin.org/articles/10.3389/fphy.2021.755139/full

Title: Erasure conversion for fault-tolerant quantum computing in alkaline earth Rydberg atom arrays
Organizations: Yale University; University of Wisconsin-Madison; Princeton University
Arrays of ultracold atoms excited to Rydberg states constitute a promising experimental platform for a range of quantum technologies. One of their main sources of error is the decay of the Rydberg state. However, in this preprint, the authors describe how 98% of these decays can be made to populate states outside of the computational basis states. Since these states can be monitored without disturbing the qubit levels, it is possible to detect which atoms have experienced an error. When errors have a known location, they are much easier to correct. The authors calculate that the surface code fault tolerance threshold for their system would be about 4% instead of the usual 1%. While related ideas have been studied for photonic qubits, this is one of the first substantial explorations for matter-based qubits. 
Link: https://scirate.com/arxiv/2201.03540

January 27, 2022