# Research Summary for June 2022

*By Dr. Chris Mansell*

### Material

**Title: Quantum Computing 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 began to be challenged by new classical heuristic approaches. Now, a programmable photonic quantum computer called Borealis has demonstrated an execution advantage over 50 million times the aforementioned processors. Time-domain multiplexing has improved scalability, while solving other technological challenges has made it less vulnerable to more recently developed classical spoofing algorithms. It can be accessed remotely over the Internet by members of the public.**Link: https://www.nature.com/articles/s41586-022-04725-x**

**Title: Negative quasi-probabilities improve phase estimation in a 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-wave plate in an optical laboratory can shift the phase of the photon by a number of degrees or radians. The goal of the rest of the experiment is to reliably estimate the phase of the photon. Finding better ways to do this has implications for various sensing applications, including gravitational wave astronomy. The accuracy of the phase estimation increases with the number of photons passing through the plate. This is fundamentally limited by the Cramér-Rao bound but by choosing to ignore some photons, the accuracy by *detected* the photon can be higher. This approach is called post-selection and it can alleviate many of the challenges that arise when taking many repeated measurements. In this work, a new filtering technique has been demonstrated experimentally, amplifying the amount of information per detected photon by more than two orders of magnitude. Unfortunately, some systematic errors have also increased.**Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.220504 **

**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; University of Ulm**

Can you hear a quantum advantage? Well, 45 people in Germany were able to correctly identify a higher proportion of sounds when recorded with a quantum-enhanced microphone than when recorded with a relatively normal laser microphone. This experiment was done in part to test an innovative quantum interferometry method that improved the sampling rate by a factor of 10,000. It also makes the improvements more relevant, so they can be appreciated by non-experts. . Microphones use a thin diaphragm that moves in response to pressure waves in the air. This movement can be converted by various means into an electrical signal. One way to monitor diaphragm movement is to use a laser. To provide a quantum enhancement, the researchers took a two-photon phase shift, which provides useful information, and imprinted it on the polarization of a single photon which could be measured much more simply than would have been possible. other. Unfortunately, the likely application of this work is not a new era of amazing quantum mechanical microphones. The test was quite contrived and is probably best suited for precision measurements of chemical reactions, biological samples, or research on atomic assemblies.**Link: https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.020358 **

**Title: Rydberg’s quantum threads for maximal independent set problems with non-planar and high-degree graphs ****Organization: Korea Higher Institute of Science and Technology**

Graph theory is used to analyze everything from traffic and social networks to Sodoku puzzles and biological systems. Networks of ultracold atoms with controllable interactions are a promising system for simulating graphs since the atoms can represent the nodes of the graph and the interactions, the edges of the graph. However, for atoms promoted to a highly excited Rydberg state, the interactions only affect other atoms within a certain radius called the blocking radius. This makes it difficult to encode non-planar graphs with long-distance 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 maximal independent sets of the associated graphs. Although there are still other technical hurdles to overcome, this demonstration is an important step towards a quantum system capable of solving difficult combinatorial optimization problems.**Link: https://www.nature.com/articles/s41567-022-01629-5**

### Software

**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. By analogy, you can consider an arrow pointing up and to the right. You are asked where it points but you are only allowed to answer with one word. You can flip a coin and, depending on its result, respond with “up” or “right”. Someone asking you the same question several times about the same arrow may infer from your answers that the arrow points diagonally. Quantum random sampling is a very important type of experiment that involves additional randomness – a random number generator is used to decide which quantum logic gates are implemented before the qubits are measured. Researchers are investigating whether this can be effectively simulated by a conventional computer. A new review article clearly summarizes this rapidly evolving field of study and offers fascinating insights into the way forward.**Link: https://arxiv.org/abs/2206.04079**

**Title: Computational advantage of a quantum superposition of Qubit gate orders****Organizations: University of Vienna; Institute of Quantum Optics and Quantum Information**

In ordinary quantum algorithms, logic gates act in a fixed order. However, the order in which gates are applied can be controlled by the state of a quantum system and that 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, in computation, the tasks known as Hadamard promise problems can only be solved if there is a specific number of gates in the circuit and the improvement brought about by the superposition of the order of the doors 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.**Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.230503**

**Title: One destined to rule them all: from adiabatic to Zeno****Organizations: Macquarie University; University of Bari; Istituto Nazionale di Fisica Nucleare; University of Trieste; Institute for Theoretische Physik, Tubingen; Waseda University**

Approximations are essential to physics and understanding their limits is crucial for their correct use. This article discusses the extremely common situation where a complicated Hamiltonian is approximated by a simpler one which still leads the quantum system to undergo a very similar time evolution. For example, the rotating wave approximation (RWA) is used to analyze the behavior of various physical qubits – from atoms to superconductors – as a function of time. Nonlinear dynamical systems theory imposes stringent limits on how RWA matches reality. The authors of the paper derive an improved bound by following the main ideas of the standard derivation, but they manage to use quantum mechanics as a linear theory. Their result is important for those working in quantum technology who seek greater precision, not only as we move towards fault-tolerant quantum computing, but also in the fields of adiabatic quantum computing and simulation. quantum.**Link: https://quantum-journal.org/papers/q-2022-06-14-737/**

**Title: Quantum advantage in learning from experiments****Organizations: Caltech; Harvard; Berkeley; Microsoft**

Quantum technology has the potential to revolutionize the way we acquire and process experimental data to learn more about the physical world. An experimental setup that transduces data from a physical system into 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 results are processed using a conventional computer. The paper proves that, in various tasks, quantum machines can learn from exponentially fewer experiments than required in conventional experiments. The exponential advantage lies 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 may be modest; for example, one can learn many non-commuting observables simultaneously by processing only two copies of the system. By conducting experiments with up to 40 superconducting qubits and 1300 quantum gates, it is demonstrated that a substantial quantum advantage can be achieved using today’s relatively noisy quantum processors. Findings highlight how quantum technology can enable powerful new strategies for learning more about nature*.***Link: https://arxiv.org/abs/2112.00778Link: https://www.science.org/doi/10.1126/science.abn7293Link: https://ai.googleblog.com/2022/06/quantum-advantage-in-learning-from.html**

July 1, 2022

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