By Dr. Chris Mansell


Title: Quantum computational advantage with a programmable photonic processor
Organizations: Xanadu; National Institute of Standards and Technology
Until the publication of this latest work, none of the photonic quantum processors that achieved a computational advantage over classical computers could be programmed and their advantages started being called into question by new classical heuristic approaches. Now, a programmable photonic quantum computer called Borealis has demonstrated a runtime advantage over 50 million times greater than the aforementioned processors. Time-domain multiplexing improved the scalability, while solving other technological challenges made it less vulnerable to the most recently developed classical spoofing algorithms. It can be remotely accessed over the internet by members of the public.

Title: Negative Quasiprobabilities Enhance Phase Estimation in Quantum-Optics Experiment 
Organizations: University of Toronto; Hitachi; NIST and University of Maryland; Harvard-Smithsonian Center for Astrophysics; Harvard University
Passing a photon through a half-waveplate in an optics laboratory can shift the phase of the photon by some number of degrees or radians. The goal of the remainder of the experiment is to reliably estimate the photon’s phase. Finding better ways to do this has knock-on implications for various sensing applications, including gravitational wave astronomy. The precision of the phase estimate increases with the number of photons that pass through the plate. This is fundamentally limited by the Cramér–Rao bound but by choosing to ignore some photons, the precision per detected photon can be higher. This approach is called postselection and it can alleviate several of the challenges that arise when taking lots of repeat measurements. In this work, a new filtering technique was experimentally demonstrated, amplifying the amount of information per detected photon by over two orders of magnitude. Unfortunately, some systematic errors were also increased.

Title: Quantum Optical Microphone in the Audio Band 
Organizations: University of Stuttgart; Center for Integrated Quantum Science and Technology; University of Cambridge; Stanford University; Olgahospital, Stuttgart; Ulm University
Can you hear a quantum advantage? Well, 45 people in Germany could correctly identify a higher proportion of sounds when they had been recorded with a quantum-enhanced microphone than when the recording was done with a comparatively normal laser microphone. This experiment was done partly to test an innovative quantum interferometry method that improved the sampling rate by a factor of 10,000. It also makes the improvements more relatable, so they can be appreciated by non-experts. Microphones employ a thin diaphragm that moves in response to pressure waves in the air. This motion can be converted by various means into an electrical signal. One way to monitor the movement of the diaphragm is with a laser. To provide a quantum enhancement, the researchers obtained a two-photon phase shift, which provides useful information, and imprinted it onto the polarisation of a single photon which could be measured far more straightforwardly than would otherwise have been possible. Sadly, the likely application of this work is not a new era of amazing quantum mechanical microphones. The test was quite contrived and is probably better suited to precision measurements of chemical reactions, biological samples or research on atomic ensembles.

Title: Rydberg Quantum Wires for Maximum Independent Set Problems with Nonplanar and High-Degree Graphs 
Organization: Korea Advanced Institute of Science and Technology
Graph theory is used to analyse everything from traffic and social networks to Sodoku puzzles and biological systems. Arrays of ultracold atoms with controllable interactions are a promising system for simulating graphs since the atoms can represent the graph‘s nodes and the interactions, the graph’s edges. However, for atoms promoted to a highly excited Rydberg state, the interactions only affect other atoms within a certain radius known as the blockade radius. This makes it challenging to encode non-planar graphs with long-ranged edges or many edges coming from the same node. In this paper, the authors introduce a quantum wire scheme based on auxiliary qubits. They experimentally construct three-dimensional configurations of atoms and by obtaining their ground states, successfully find the maximum independent sets of the associated graphs. While there are still further technical hurdles to overcome, this demonstration is a significant step towards a quantum system being able to solve difficult combinatorial optimisation problems.


Title: Computational advantage of quantum random sampling
Organizations: NIST and University of Maryland; Freie Universität Berlin; Helmholtz-Zentrum Berlin für Materialien und Energie; Fraunhofer Heinrich Hertz Institute
In quantum mechanics, measurements are probabilistic. As an analogy, you can consider an arrow that is pointing up and to the right. You are asked where it is pointing but are only allowed to reply with a single word. You might flip a coin and based on its result, respond with either “up” or “right.” Someone asking you the same question many times about the same arrow will eventually be able to infer from your responses that the arrow is pointing diagonally. Quantum random sampling is a very important type of experiment that involves further randomness – a random number generator is used to decide which quantum logic gates are implemented before the qubits are measured. Researchers are investigating whether or not this can be efficiently simulated by a classical computer. A new review article clearly summarises this rapidly evolving field of study and provides some fascinating perspectives on the way forward.

Title: Computational Advantage from a Quantum Superposition of Qubit Gate Orders
Organizations: University of Vienna; Institute for Quantum Optics and Quantum Information
In ordinary quantum algorithms, the logic gates act in a fixed order. However, the order in which the gates are applied can be controlled by the state of a quantum system and this system can be in a quantum superposition. Therefore, it is possible to apply gates in a superposition of different orders. For communication tasks, quantum protocols with an indefinite causal order can lead to an exponential advantage. However, when it comes to computation, the tasks known as Hadamard promise problems can only be solved if there are a specific number of gates in the circuit and the improvement provided by superposing the gate order is rather modest. In this recent research, these problems are extended so that they can have any number of gates. Importantly, the extension only requires qubits, as opposed to high-dimensional systems, which means experimental demonstrations could follow.

Title: One bound to rule them all: from Adiabatic to Zeno
Organizations: Macquarie University; Università di Bari; Istituto Nazionale di Fisica Nucleare; Università di Trieste; Institut für Theoretische Physik, Tübingen; Waseda University
Approximations are essential to physics and understanding their limits is crucial to their correct usage. This paper deals with the extremely common situation where a complicated Hamiltonian is approximated by a simpler one that still leads to the quantum system undergoing a very similar time evolution. For example, the rotating-wave approximation (RWA) is used to analyse how various physical qubits – from atoms to superconductors – behave as a function of time. The theory of non-linear dynamical systems puts rigorous bounds on how well the RWA matches up with reality. The authors of the paper derive an improved bound by following the main ideas of the standard derivation but they manage to make use of quantum mechanics being a linear theory. Their result is important for those working in quantum technology who strive for greater precision, not only as we progress towards fault-tolerant quantum computation but also in the fields of adiabatic quantum computing and quantum simulation.

Title: Quantum advantage in learning from experiments
Organizations: Caltech; Harvard; Berkeley; Microsoft
Quantum technology has the potential to revolutionize how we acquire and process experimental data to learn about the physical world. An experimental setup that transduces data from a physical system to a stable quantum memory, and processes that data using a quantum computer, could have significant advantages over conventional experiments in which the physical system is measured and the outcomes are processed using a classical computer. The paper proves that, in various tasks, quantum machines can learn from exponentially fewer experiments than those required in conventional experiments. The exponential advantage holds in predicting properties of physical systems, performing quantum principal component analysis on noisy states, and learning approximate models of physical dynamics. In some tasks, the quantum processing needed to achieve the exponential advantage can be modest; for example, one can simultaneously learn about many noncommuting observables by processing only two copies of the system. Conducting experiments with up to 40 superconducting qubits and 1300 quantum gates, it is demonstrated that a substantial quantum advantage can be realized using today’s relatively noisy quantum processors. The results highlight how quantum technology can enable powerful new strategies to learn about nature.

July 1, 2022