By Dr. Chris Mansell


Title: Quantum and non-local effects offer over 40 dB noise resilience advantage towards quantum lidar 
Organization: University of Toronto

LiDAR systems, such as those in some self-driving cars, can measure distances by shining a laser onto an object and measuring the time for the reflected light to return to a receiver. In this paper, quantum correlations between the times of the probe- and reference-photons were used to improve the receiver’s signal-to-noise ratio by 43 decibels. The demonstrated system could also tolerate 1000 times as much background noise as a comparable classical set-up. These impressive results were obtained in a laboratory setting but given the system’s performance and noise-resilience, it seems plausible that future upgrades could be tested out in the real world.

Title: A shuttling-based two-qubit logic gate for linking distant silicon quantum processors 
Organizations: RIKEN; Delft University of Technology; Netherlands Organization for Applied Scientific Research (TNO)

Quantum logic gates between silicon quantum dots typically rely on a short-range exchange coupling between nearest neighbours. This work demonstrated a controlled-phase gate between a stationary qubit and a qubit that has been moved from one dot to the next by a phase coherent spin shuttling process. The fidelity was 93% but this could potentially be increased by an additional barrier pulse and more frequent auto-calibration. With these improvements in place, the next step will be to see if the qubits can be moved over longer distances.

Title: Universal control of a six-qubit quantum processor in silicon 
Organizations: Delft University of Technology; Netherlands Organization for Applied Scientific Research (TNO)

Experiments involving four or fewer spin qubits in silicon quantum dots typically optimise either the fidelity of the quantum logic gates or the initialisation and measurement fidelities. In this impressive paper, the researchers created six spin qubits in a linear quantum dot array and introduced new techniques that allow the aforementioned fidelities to simultaneously reach high values. They demonstrated universal control over these qubits by preparing maximally entangled states and gave plans for further improving and scaling their device.

Title: On the Emerging Potential of Quantum Annealing Hardware for Combinatorial Optimization
Organizations: Los Alamos National Laboratory; University of New Mexico

D-Wave Systems’ most recent quantum annealing device can solve sparse unconstrained quadratic optimization problems with over 5,000 binary decision variables and 40,000 quadratic terms. This study shows that there are classes of contrived problems where this quantum annealer provides an order-of-magnitude run time benefit over state-of-the-art classical solution methods. The work has a few limitations that are openly discussed but the results hold even when accounting for practicalities and overheads. 

Title: Protecting Fiber-Optic Quantum Key Distribution Sources against Light-Injection Attacks
Organizations: Russian Quantum Center; National University of Science and Technology MISiS; University of Waterloo; Mahidol University; Quantum technology foundation (Thailand); ITMO University; SMARTS-Quanttelecom; University of Science and Technology of China; National University of Defense Technology, China

Device-dependent quantum key distribution requires a light source that operates within well-defined parameters. If eavesdroppers inject bright light into the system, it can change the properties of the light source and thus, ruin the security of the set-up. This paper proposed and tested a countermeasure in the form of a component that under lower levels of illumination, protected the light source and under higher levels, permanently and securely ended the protocol. Importantly for the practicality of the proposal, the crucial component could be a standard fiber-optic isolator or a simple circulator. 

Title: Testing platform-independent quantum error mitigation on noisy quantum computers
Organizations: Unitary Fund;  École Polytechnique Fédérale de Lausanne (EPFL); Goldman Sachs & Co.

Zero-noise extrapolation and probabilistic error cancellation are two important quantum error mitigation techniques. In this paper, they were applied to two benchmark problems that were each run on IBM, IonQ and Rigetti quantum computers. The benefit that these techniques bring was characterised by an improvement factor. The results varied depending on the underlying hardware but the key take-away was that on average, error mitigation is beneficial compared to no mitigation. The authors expect that error mitigation will feature in nearly all the experiments in the NISQ era.


Title: A streamlined quantum algorithm for topological data analysis with exponentially fewer qubits 
Organizations: AWS; California Institute of Technology; Alfréd Rényi Instit
ute of Mathematics; Imperial College
Analysis of the topological features of a dataset can be useful in many data science and machine learning contexts. This paper presents a quantum algorithm that improves upon its predecessors by being polynomially quicker and needing exponentially fewer qubits. It also has a roughly quintic speedup over rigorous classical algorithms and an approximately quadratic speedup over heuristic classical algorithms. Despite this, the authors suspect that the conditions required for their algorithm to show an advantage may not occur very frequently in real-world datasets.

Title: Quantum Computation of Molecular Structure Using Data from Challenging-To-Classically-Simulate Nuclear Magnetic Resonance Experiments 
Organizations: Google; University of Maryland

Nuclear magnetic resonance (NMR) experiments are used by chemists and biologists to investigate the properties of molecules. However, the data that get generated are often difficult for classical computers to analyse. In this paper, a quantum algorithm is presented for using such data to infer molecular nuclear spin Hamiltonians. The algorithm can be run on both present-day NISQ devices and future fault-tolerant quantum computers. The researchers used the protein ubiquitin as an example and hope that their work will further the development of new NMR-based techniques for molecular structure analysis. 

Title: Portfolio Optimization via Quantum Zeno Dynamics on a Quantum Processor
Organizations: JPMorgan Chase

The quantum Zeno effect is named after Zeno’s arrow paradox and can be implemented using repeated projective measurements. In this preprint on optimising portfolios with quantum algorithms, this effect is used to enforce multiple constraints. This is done by introducing penalty terms to the objective function and slack variables to the inequality constraints. The methods require only a few auxiliary qubits and no post-selection yet they can be incorporated into the QAOA algorithm and variational quantum algorithms to achieve high-quality, valid solutions. This was tested on the Quantinuum H1-2 trapped-ion quantum processor with two-qubit gate depths of up to 148.

Title: The Complexity of NISQ
Organizations: UC Berkeley; Harvard University; California Institute of Technology; Microsoft Research

In this work, the authors define a complexity class intended to capture the problems that can be efficiently solved by a classical computer combined with a NISQ device. Their analysis takes place in the circuit model of quantum computation where all the key steps are noisy: the initialisation of the qubits; the implementation of the logic gates; and the measurement process. Using the tools of quantum query complexity, they demonstrate that this complexity class is strictly more powerful than the complexity class describing classical computation and strictly less powerful than the one for fault-tolerant quantum computation. Overall, their mathematical framework allowed them to draw very general conclusions about the current era of NISQ computation.

Title: The Quantum Fourier Transform Has Small Entanglement
Organizations: University of California, Irvine; California Institute of Technology; Flatiron Institute

Fourier transforms are extremely important in mathematics, physics, computer science and engineering. In quantum computing, the Quantum Fourier Transform (QFT) plays a starring role in quantum phase estimation and in Shor’s algorithm for integer factorisation. It was already known that the QFT has maximal operator entanglement but the authors of this paper showed that this is mainly due to a step in the process where the order of the qubits is reversed. They found that this step isn’t needed and that without it, the QFT has a different entanglement structure that makes it easier to simulate on a classical computer. They called this a “quantum-inspired classical algorithm” and showed that it can outperform the ubiquitous Fast Fourier Transform.  

Title: Exponentially tighter bounds on limitations of quantum error mitigation 
Organizations: Freie Universität Berlin; University of Copenhagen; ENS Lyon; Helmholtz-Zentrum Berlin für Materialien und Energie

Quantum error mitigation (QEM) schemes are of great interest because they require few additional quantum resources and yet promise to enable useful quantum computations in the NISQ era. Their advantages have been demonstrated on the small quantum processors that are available today. However, theoretical results can let us know what to expect as the devices grow in their quantum volume and this work proves that the QEM protocols (that fall into its framework) perform exponentially poorly in both circuit depth and circuit width. The theory is a worst-case analysis and so further research is needed to see if the results apply to typical quantum circuits. 

October 29, 2022