By Dr. Chris Mansell

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

Title: High-fidelity laser-free universal control of two trapped ion qubits
Organization: NIST and others
Ion trap quantum computers employing lasers have the highest fidelity logic gates out of all the experimental platforms currently available and arguably have more concrete plans for scalability than their competitors. In this work, microwave technology initially developed for wireless communication is used to perform laser-free logic gates with just as high fidelity. By removing the challenges involved with aligning large numbers of lasers, the scaling prospects of ion traps are considerably improved. You can view the paper here. https://arxiv.org/abs/2102.12533

Title: Realization of a multi-node quantum network of remote solid-state qubits
Organization: QuTech and Delft University of Technology
Solid state qubits have been improving by orders of magnitude in recent years: the time to create entanglement has fallen and the coherence time has risen. By putting these together, the researchers have created a three-node quantum network. In the future, this technology could be used at quantum data centres and eventually may lead to a quantum internet. You can view the paper here. https://arxiv.org/abs/2102.04471 

Title: Massively parallel ultrafast random bit generation with a chip-scale laser
Organizations: Yale University and others
Random number generators are crucial for cryptography, machine learning and numerical methods. The authors use a broad-area, multimode laser to produce numbers at  significantly faster rates than before. These numbers pass comprehensive tests for randomness. Further work is required for this to become a commercial device but you can view the paper here https://arxiv.org/abs/2004.07157 and a commentary here https://cpb-us-w2.wpmucdn.com/u.osu.edu/dist/7/38882/files/2021/02/Science371-889-2021.pdf

Title: Quantum circuits with many photons on a programmable nanophotonic chip
Organizations: Xanadu and NIST
The researchers overcame various engineering challenges to create a full-stack, remotely programmable and potentially highly scalable nanophotonic chip which uses strongly squeezed light as qubits. They verified their device by performing three different quantum algorithms. You can view the paper here https://arxiv.org/abs/2103.02109 and a short explanatory video here https://www.xanadu.ai/hardware 

Title: A semi-agnostic ansatz with variable structure for quantum machine learning
Organizations: Universitat Autonoma de Barcelona, Los Alamos National Laboratory, Sandbox @Alphabet and University of Waterloo
Before a parametrised quantum circuit can be optimised, the layout of the gates, called the ansatz, must be decided. Keeping the circuit depth low helps with the training of the parameters because noise is less of an issue. For circuits with the same number of gates, the variable ansatz devised in this paper achieved better performance than the Hardware Efficient Ansatz across a variety of tasks. You can view the paper here. https://arxiv.org/abs/2103.06712

Title: Speeding up quantum perceptron via shortcuts to adiabaticity
Organizations: University of the Basque Country, IQM and others 
Quantum systems evolve in a linear, unitary manner but neural networks require non-linear activation functions. By considering the application of inversely engineered control fields, the authors figured out how to drive sigmoid-like responses in a faster and more robust way than with any other known method. They say that nitrogen vacancy centres could be the best qubits, or quantum neurons, for an experimental implementation of their protocol. You can view the paper here. https://www.nature.com/articles/s41598-021-85208-3

Title: Efficient estimation of Pauli observables by derandomization
Organizations: Caltech, AWS, Johannes Kepler University 
Shadow tomography is a new idea where the goal is to find an efficient way to predict some important properties of a quantum state without having to fully characterise it. Focussing on Pauli observables, the authors devise a deterministic protocol that in their numerical experiments, outperforms some popular randomised protocols by an order of magnitude. You can view the paper here. https://arxiv.org/abs/2103.07510

Title: Experimental quantum speed-up in reinforcement learning agents
Organizations: University of Vienna, NOKIA and others
In reinforcement learning, a major sub field of artificial intelligence, an agent interacts with its environment and uses feedback to learn better behaviours. In this paper, the authors investigate the learning rate when the interactions take place through a quantum communication channel. They implement their protocol on a nanophotonic processor and find systematic advantages. You can view the paper here. https://arxiv.org/abs/2103.06294

Title: A Quantum Algorithm for the Sensitivity Analysis of Business Risks
Organizations: JoS QUANTUM and Deutsche Börse Group
In a very clear paper, the authors describe a classical model that takes the  Deutsche Börse Group days to weeks of calculation time to test the sensitivity to each input. It would take an infeasibly long time to examine every possible pair or triplet of inputs. They tested a quantum circuit for the same task by simulating up to 20 qubits and found a quadratic improvement. You can view the paper here. https://arxiv.org/abs/2103.05475

Title: A four-qubit germanium quantum processor
Organizations: QuTech, Delft University of Technology and Netherlands Organisation for Applied Scientific Research 
The researchers constructed a device with four germanium quantum dots that acted as qubits. They performed single-qubit gates with fidelities above 99% and demonstrated full control over the different qubits by using dynamical decoupling techniques and multi-qubit logic gates to prepare a four-qubit GHZ state. You can view the paper here https://www.nature.com/articles/s41586-021-03332-6 or here https://arxiv.org/abs/2009.04268

Title: Removing leakage-induced correlated errors in superconducting quantum error correction
Organizations: University of California, Google, Johannes Kepler University and University of Texas
During the operation of a quantum computer, states outside of the computational basis states can become excited. Often this happens in a correlated way, which is an issue for error correction protocols. In this paper, a reset procedure that suppressed these errors was experimentally implemented in a superconducting quantum processor. You can view the paper here. https://arxiv.org/abs/2102.06131

March 29, 2021