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: Accurately computing electronic properties of a quantum ring
Organizations: Google Quantum AI; University of Massachusetts; University of California, Santa Barbara; University of California, Riverside; University of Chicago
Small errors in the numerical analysis of quantum materials can lead to large deviations in our predictions of their most interesting observable properties. The key insight of this paper is to perform experimental work in the Fourier domain. They first do this with individual pairs of superconducting qubits and then with a total of 54 qubits. They achieve unprecedented computational accuracy and could beat the current state-of-the-art classical methods in the very near future.
https://www.nature.com/articles/s41586-021-03576-2

Title: Observation of separated dynamics of charge and spin in the Fermi-Hubbard model
Organizations: Google AI Quantum and collaborators
The Sycamore quantum processor, which famously demonstrated quantum supremacy in 2019, is now being used to investigate interacting electrons in the hope that it might one day yield insights into phenomena such as high-temperature superconductivity. By using a new calibration technique, the researchers from Google increased the circuit depths by an order of magnitude. They observed interesting quasiparticles dynamics and since digital quantum gates can be implemented, there is greater potential for future work than with analogue quantum simulators. 
https://arxiv.org/abs/2010.07965

Title: Full control of superconducting qubits with combined on-chip microwave and flux lines
Organization: Rigetti
Routing control lines without adding design complexity is an important step for scaling superconducting quantum devices. This work introduces a method to integrate a microwave line and a flux line into a single “XYZ line”. This approach can allow performing high-fidelity single and two-qubit gates with XYZ lines.
https://arxiv.org/abs/2107.06245

Title: Heralded entanglement distribution between two absorptive quantum memories
Organization: University of Science and Technology of China (USTC)
Quantum networks require components called quantum repeaters. If they can absorb and emit photons, then they can act as a read-write memory and help establish links between remote parts of the network. A team from USTC created a repeater, nicknamed the magpie bridge, that could store multiplexed photons, provide heralded signals of entanglement and operate at telecommunications wavelengths. In the future, they will greatly enhance the entanglement distribution rate by updating the light source to a deterministic entanglement source.
https://www.nature.com/articles/s41586-021-03505-3

Title: Piezoacoustics for precision control of electrons floating on helium
Organizations: Michigan State University; EeroQ Corp
The surface of superfluid helium is a pristine substrate free of the defects and imperfections present in almost all other materials. In a vacuum, electrons that could act as qubits float about 10 nm above the superfluid surface. This research is the first acoustoelectric experiment on helium. The demonstration of controlled charge transport means there are interesting experiments to come for this unique low-dimensional electron system.
https://www.nature.com/articles/s41467-021-24452-7 and https://eeroq.com/

Title: Realization of real-time fault-tolerant quantum error correction
Organization: Honeywell Quantum Solutions
When detecting errors is vital, classical computers can use triplets of bits to store and manipulate the information that would usually be encoded into one bit. If one of the three bits disagree with the other two, it was probably the one that picked up an error. We say that three physical bits make a logical bit and even though it isn’t as straightforward in the quantum case, it is central to quantum error correction protocols. In this paper, researchers from Honeywell perform numerous operations on a single logical qubit that they created from seven trapped ion physical qubits. Continued hardware improvements and the creation of more than one logical qubit are important next steps.
https://arxiv.org/pdf/2107.07505.pdf 

Title: A universal fully reconfigurable 12-mode quantum photonic processor
Organizations: QuiX BV; Mesa+ Institute for Nanotechnology
For a quantum optics experiment to achieve a useful quantum advantage, it must be large and reconfigurable. To date, a quantum advantage involving 100 optical modes has been performed on an optical bench that could only be reconfigured manually by skilled experimentalists. Building on their 2019 quantum photonic processor with 8 modes, QuiX Quantum have now created a 12-mode, reconfigurable, universal, high fidelity, low loss, fully tunable linear interferometer. It can be completely controlled by a convenient software interface, which will allow it to explore a wide range of applications.
https://iopscience.iop.org/article/10.1088/2633-4356/ac168c

Title: Quantum Oscillator Noise Spectroscopy via Displaced Cat States
Organizations: The University of Sydney; Q-CTRL
Performing high-fidelity logic gates on trapped ion qubits requires exquisite control of their joint motion. Cat states, named after Schrödinger’s cat, are exceptionally sensitive to any noise that may disturb the ions’ oscillation. This paper uses this sensitivity, along with some clever tricks and algorithms, to precisely probe and pinpoint the sources of noise. Going forwards, this information may enable the noise to be considerably minimised. 
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.250506

Software

Title: Compact fermion to qubit mappings
Organizations: Phasecraft; University College London
Simulating electrons is one of the most promising and long-standing motivations for quantum technology. Electrons have antisymmetric exchange statistics, which makes it hard for them to be represented by qubits in an efficient way. This work presents a new state-of-the-art modelling technique that will allow near-term quantum computers to get closer to producing genuinely practical results.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.035118

Title: Provably efficient machine learning for quantum many-body problems
Organizations: Caltech; Johannes Kepler University; AWS; National Institute of Standards and Technology; University of Maryland
Classical shadows are succinct classical descriptions of quantum many-body states. They can be processed in simple ways to predict certain properties of those quantum systems. The authors of this paper proved that more advanced processing, involving neural networks and kernel methods, enables the predictions of ground state properties and the classification of quantum phases. This approach could be used to predict the features of unstudied quantum systems, which, in the future, could help material science innovation and drug discovery.
>https://arxiv.org/abs/2106.12627

Title: Quantum advantage for computations with limited space
Organization: IBM
Restricted models of quantum computation have played an important role in our understanding of quantum mechanics. In this work, the limitation is that there is a single target qubit for the controlled quantum logic gates. Comparing this to a classical computer with a similar constraint, the algorithmic success probability of the quantum algorithms exceeds that of the best possible classical algorithms. The authors used quantum signal processing techniques to mathematically prove this before experimentally demonstrating it on the ibmq_berlin device. Researchers will now want to explore the interplay and trade-offs between this and other quantum advantages. 
https://www.nature.com/articles/s41567-021-01271-7

Title: Filtering variational quantum algorithms for combinatorial optimization
Organization: Cambridge Quantum Computing Limited
Optimization problems that are encountered in several industries can be encoded into quantum systems so that their low energy states represent good solutions to the problem. In particular, the lowest energy state corresponds to the best solution. This work introduces the concept of quantum operators that “filter out” the high energy states. This new idea was run on the Honeywell trapped ion quantum computer and it outperformed all the leading quantum optimization algorithms. It will be interesting to see how far these new insights can be pushed by quantum technology researchers.
https://arxiv.org/abs/2106.10055

July 29, 2021