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.


Title: Quantum Chaos = Volume-Law Spatiotemporal Entanglement
Organizations: Monash University; Centre for Quantum Technology, Transport for New South Wales
In classical mechanics, there is a well-developed theoretical framework for investigating chaotic systems. The central concept is that a small perturbation, such as the flap of a butterfly’s wings, can lead to an outsized effect, such as a storm several days later in another part of the world. Chaos in quantum mechanical systems has not been formalised to the same extent. However, these researchers propose a unifying, operational definition. They show that quantum processes that create volume-law entangled states display chaotic behaviours, like highly non-local phenomena deterministically arising from – and very sensitively depending on – small, local disturbances in the past. This new, insightful connection could be helpful for various many-body physics problems. 

Title: Learning to predict arbitrary quantum processes 
Organizations: Caltech; UC Berkeley; AWS
Quantum physics experiments often involve three stages: preparing an initial quantum state; letting it evolve in time; and finally, obtaining a measurement result. Could a supervised machine learning (ML) model be given training data that follows a similar format, such as a classical description of the initial quantum state paired with the outcome of a measurement on the time-evolved quantum state? The paper describes a computationally efficient ML algorithm that can be trained on such a dataset even when the time-evolution consists of exponentially many quantum logic gates. The algorithm is related to classical shadow tomography and relies on some new mathematical derivations. The results of this work could be employed in the investigations of many different quantum technologies. 

Title: Strongly Contracted N-Electron Valence State Perturbation Theory Using Reduced Density Matrices from a Quantum Computer
Organization: Quantinuum
In this paper, a new hybrid quantum-classical algorithm for solving electronic structure problems is presented. It works by considering only the most strongly correlated orbitals in the quantum processor. To do this, the quantum states are prepared according to the variational quantum eigensolver algorithm and then reduced density matrices are measured. Combined with classical processing, the overall approach achieved results that agreed with those of earlier quantum chemistry methods. Importantly, the hybrid approach has excellent scaling, so as better quantum hardware gets developed in the coming years, it should be able to overtake the completely classical methods. 

Title: Perspective on the Current State-of-the-Art of Quantum Computing for Drug Discovery Applications
Organizations: Riverlane; Astex Pharmaceuticals
This perspective on quantum chemistry focusses on how electronic structure problems being solved on a quantum computer could impact the pharmaceutical industry. It compares some important algorithms – quantum phase estimation versus the variational quantum eigensolver and qubitization versus Trotterization – and in both these cases, concludes that the former has better scaling than the latter. It examines a drug called Ibrutinib that treats non-Hodgkin lymphoma and estimates that by using today’s best techniques, error-corrected quantum calculations could take just a few days (instead of over a thousand years if previous methods were used).

Title: A polynomial-time classical algorithm for noisy random circuit sampling
Organizations: The Hebrew University; Harvard University; UC Berkeley
Random quantum circuits (RQCs) have recently been the basis of quantum supremacy experiments but since we are in the NISQ era, the circuits were noisy. Classical algorithms that give the same output as a noisy RQC have been thoroughly investigated and this paper presents a new one that has a polynomial asymptotic runtime. This implies that noisy RQC experiments are not a scalable way to demonstrate quantum supremacy. This is a highly significant implication. Even so, the algorithm has room for improvement because when its concrete, non-asymptotic runtimes are estimated, they do not challenge either the leading quantum processors or the existing classical spoofing algorithms.

Titles: Universal Parity Quantum Computing; Applications of universal parity quantum computation
Organizations: University of Innsbruck; Parity Quantum Computing GmbH 
In the “Parity Quantum Computing” architecture, some logical qubits are encoded into a larger number of physical qubits. The way this is done goes against the conventional approach of a logical one (zero) being redundantly encoded as an odd number of physical ones (zeroes). Instead, any given pair of logical qubits is associated with a single physical qubit. The parity, considered in the Pauli Z direction, of the pair determines the state of the physical qubit. This approach has numerous advantages (such as the logical qubits having all-to-all connectivity) but until now, it could only be applied to combinatorial optimization problems. The first of these two, new papers shows how universal quantum computation can be achieved with a slight modification to this architecture. The second paper analyses the gate count associated with implementing Shor’s algorithm in this way and finds some notable benefits.


Title: Robust multi-qubit quantum network node with integrated error detection
Organizations: Harvard University; AWS; Delft University of Technology; Universität Hamburg; The Hamburg Centre for Ultrafast Imaging
Quantum repeaters are important for long-distance quantum communication and the quantum internet. They are devices that perform entanglement distribution, entanglement swapping and entanglement purification. It is important that they interface with photons efficiently and have extended memory times. In this paper, silicon-vacancy centers in diamond nanophotonic cavities were realised that had a memory of over two seconds and that could perform entangling electron-photon gates at temperatures up to 1.5 Kelvin and entangling nucleus-photon gates up to 4.3 Kelvin. This is an important step in the development of scalable quantum repeaters.

Title: Unimon qubit 
Organizations: IQM; Aalto University; VTT Technical Research Centre of Finland Ltd.; QTF Centre of Excellence
Transmon and fluxonium qubits are two of the most popular types of superconducting qubit. A transmon qubit consists of a Josephson junction with a shunt capacitor in parallel. Fluxonium qubits were developed by adding in an additional inductor. In this paper, a “unimon” qubit is introduced where the inductive energy and the Josephson energy are both about 100 times larger than the energy of the capacitor. This design leads to a number of desirable properties. A high fidelity, 13 nanosecond, single-qubit gate is demonstrated. Future work will focus on reducing dielectric losses. 

Title: Hardware optimized parity check gates for superconducting surface codes
Organizations: Rigetti Computing; Goldman, Sachs & Co.
One route to achieving fault-tolerant quantum computation is to co-design the computing hardware and the error-correction software. This paper presents the design of a five-qubit logic gate based on many-body interactions between superconducting transmon qubits. The gate is tailored to be used for stabilizer-type measurements in a surface code error-correction protocol. The new ‘Hardware Optimized Parity’ gates are simulated and improvements to the error-correction thresholds are found, which is a strong indication of the usefulness of this approach. 

Title: Scalable algorithm simplification using quantum AND logic
Organizations: Southern University of Science and Technology, Shenzhen; International Quantum Academy, Shenzhen; Chinese Academy of Sciences; University of Chinese Academy of Sciences
Quantum computations are commonly decomposed into a sequence of single- and two-qubit gates taken from a universal gate set. However, schemes that use ancilla qubits, multi-qubit logic gates or ancillary quantum states are also being researched. The goal is to compile a quantum circuit that is resource-efficient for the given hardware. In this paper, Grover’s algorithm is expressed as a low-depth circuit of high-fidelity gates that employ an extra quantum state for temporary information storage. The algorithm was performed on a superconducting quantum processor but it could straightforwardly be adapted to other quantum computing platforms.  

Title: Demonstration of a Quantum Gate Using Electromagnetically Induced Transparency
Organization: University of Strathclyde
Electromagnetically induced transparency (EIT) is a coherent phenomenon involving the interference of transitions in a three-level quantum system. In 2009, a theoretical proposal was put forward to use EIT and Rydberg blockade to implement high-fidelity multi-qubit gates in arrays of neutral atoms. In this latest paper, the scheme has been experimentally demonstrated. The achieved fidelities were limited by the available laser power as well as by laser phase noise. If known methods to improve the laser system could be implemented, the gate could be used in a variety of interesting quantum protocols.

December 1, 2022