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.

Software

Title: The boundary for quantum advantage in Gaussian boson sampling
Organizations: University of Bristol; Imperial College London; Hewlett Packard

Last month, we saw at least two notable results regarding the Gaussian boson sampling (GBS) protocol. While one set of researchers found quicker ways to classically simulate its results, another was busy tightening up the quantum protocol, effectively moving the goal posts to make it less susceptible to any sort of classical simulation. New results are still coming thick and fast. This time, a GBS experiment that seemed to make classical computers need 600 million years to simulate it has now been simulated in just 73 days. This is quicker by a factor of 3 billion. While there are a few useful applications of GBS research, a great deal of its importance lies in how it could act as a leading indicator for the quantum technology industry as a whole: if quantum experiments can arrive at results that we have no other way of obtaining, then they are truly worth doing because it may not be long until quantum devices start having practical impacts on the world.
Link: https://www.science.org/doi/10.1126/sciadv.abl9236

Title: A quantum-inspired approach to exploit turbulence structures
Organizations: University of Oxford; Cambridge Quantum Computing Limited; University of Bath; University of Pittsburgh; National University of Singapore; Universität Hamburg

Turbulence occurs as air flows over moving vehicles, as blood courses through our bodies and as industrial fluids pass through pipes. Turbulence is a scourge to controlled nuclear fusion, just as decoherence is to quantum technologies. In a paper that appeared when a new world record for the amount of energy extracted from a fusion power plant was set, researchers used tools from quantum many-body physics to analyse turbulence in a novel way. They ran their algorithm on a classical computer to demonstrate its superior efficiency over direct numerical simulation and then showed how it could be adapted to run even faster on a quantum computer. Beyond the ambitious goal of commercial fusion, these improvements to the way turbulence is modelled have the potential to help numerous industries where turbulent fluid flow is an issue.
Link: https://www.nature.com/articles/s43588-021-00181-1

Title: Quantum algorithm for credit valuation adjustments
Organizations: Zapata Computing; BBVA Corporate & Investment Banking; IBM
Credit valuation adjustments (CVAs) are computationally expensive simulations that banks are legally required to perform. Many stochastic paths are sampled in order to statistically analyse the risk of counterparty default. The aim is to avoid the events that led to the Great Recession. In this study, the authors developed a flexible quantum algorithm for calculating CVAs and identified possible ways to keep the circuit depth low. They also dove into the details of how a specific implementation would run in practice. Further developments are needed for quantum approaches to make headway in the finance industry but in this highly active research area, there are sure to be more results soon.
Link: https://iopscience.iop.org/article/10.1088/1367-2630/ac5003

Title: Noisy intermediate-scale quantum (NISQ) algorithms
Organizations: National University of Singapore; University of Toronto; Imperial College; Harvard University; Massachusetts Institute of Technology; CNRS-UNS-NUS-NTU International Joint Research Unit; Nanyang Technological University; Vector Institute for Artificial Intelligence; Canadian Institute for Advanced Research
This review paper aims to describe the different algorithms that could produce important results in the NISQ era. It covers variational quantum algorithms as well as those inspired by hybrid and analog approaches to quantum computing. Along the way, it highlights both the theoretical and the experimental challenges these algorithms face. The survey discusses the plethora of applications that are currently being explored in fields like machine learning, optimisation and chemistry. Methods for comparing the performance of quantum processors and a list of open-source quantum software tools are also given. Finally, a thoughtful and thorough assessment is made on the near-term goals of NISQ research and the longer-term hopes for achieving fault tolerance.
Link: https://journals.aps.org/rmp/abstract/10.1103/RevModPhys.94.015004

Hardware

Title: Significant loophole-free test of Kochen-Specker contextuality using two species of atomic ions
Organizations: Tsinghua University; Southern University of Science and Technology; Duke University; California Institute of Technology; Beijing Academy of Quantum Information Sciences; Universidad de Sevilla

The Kochen-Specker theorem limits what types of hidden-variable theories are consistent with the predictions of quantum mechanics. More specifically, it excludes theories where the hidden variables do not depend on the context in which a measurement is taking place. It is similar to how Bell’s theorem ruled out local explanations of quantum physics. Various experiments to date have investigated contextuality but none have been free of loopholes. This new report is the first conclusive confirmation of the theorem. The experiment was performed with trapped ytterbium and barium ions that could be measured with 100% efficiency. That is, no attempted measurements failed to give a result throughout the entire experiment. The work has practical applications because demonstrating noncontextuality could be a useful way to certify quantum computers.
Link: https://www.science.org/doi/10.1126/sciadv.abk1660

Title: Quantum Optimization of Maximum Independent Set using Rydberg Atom Arrays
Organizations: Harvard University; QuEra Computing; University of Waterloo; Perimeter Institute for Theoretical Physics; Google; Massachusetts Institute of Technology; Institute for Advanced Study; California Institute of Technology; University of Innsbruck; Austrian Academy of Sciences
In this impressive experimental research, between 39 and 289 ultracold rubidium atoms were individually trapped in optical tweezers in various two-dimensional arrangements in order to represent sets of nodes and edges known as graphs. An important problem in graph theory – that sounds abstract but has several applications – is the Maximum Independent Set (MIS) problem. The goal is to find the largest subset of vertices that are not joined by edges to each other. Exciting nearby atoms to states with large principal quantum numbers, known as Rydberg states, allows the phenomenon of Rydberg blockade to occur. At the end of the procedure, the ground state of the system encodes the solution to the MIS problem. The most promising results were for the graphs that classical solvers find the hardest. For the quantum approach to be competitive, the coherence time of the atoms would need to be longer and the system size larger.
Link: https://arxiv.org/abs/2202.09372

Title (1): Resource-Efficient Dissipative Entanglement of Two Trapped-Ion Qubits
Organizations (1): National Institute of Standards and Technology; University of Colorado; Universität Kassel; ETH Zürich; Freie Universität Berlin
Link (1): https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.080502
Title (2): Generation of a Maximally Entangled State Using Collective Optical Pumping
Organization (2): ETH Zürich
Link (2): https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.080503

Non-classical states can be created by employing unitary transformations or by engineering dissipative dynamics. Various protocols for the latter approach have been proposed and implemented. Their main advantage is a greater robustness to many forms of experimental imperfections, such as fluctuating control fields. In these two papers, dissipation is used to create entangled pairs of trapped ions. The fidelities were reported to be 93% for two calcium ions and 95% for two beryllium ions. Both sets of researchers expect these fidelities to increase when fairly standard improvements are made to their set-ups. The protocols that were demonstrated could be adapted to several other types of quantum hardware, such as nitrogen-vacancy centers, neutral atom platforms or superconductors.

Title: Superresolution Microscopy of Optical Fields Using Tweezer-Trapped Single Atoms
Organizations: University of California; Max-Planck-Institut für Quantenoptik; Munich Center for Quantum Science and Technology; Lawrence Berkeley National Laboratory

In this impressive experimental research, between 39 and 289 ultracold rubidium atoms were individually trapped in optical tweezers in various two-dimensional arrangements in order to represent sets of nodes and edges known as graphs. An important problem in graph theory – that sounds abstract but has several applications – is the Maximum Independent Set (MIS) problem. The goal is to find the largest subset of vertices that are not joined by edges to each other. Exciting nearby atoms to states with large principal quantum numbers, known as Rydberg states, allows a phenomenon of Rydberg blockade to occur. At the end of the procedure, the ground state of the system encodes the solution to the MIS problem. The most promising results were for the graphs that classical solvers find the hardest. For the quantum approach to be competitive, the coherence time of the atoms would need to be longer and the system size larger.
Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.083201

February 26, 2022