Terra Quantum Senior Scientific Writer Dr. Chris Mansell
Below is a summary of some interesting research papers on quantum computing and communications that we have seen over the past month.
Title: Classical Stiffness Calibration of Quantum Approximation Optimization Algorithms
Organization: University of California, Berkeley.Lawrence Berkeley National Laboratory; Righetti Computing
An increasingly popular method for classically simulating quantum circuits is that of tensor networks. This paper focuses on a type of tensor network called matrix product state (MPS). As the state of the quantum circuit evolves, it becomes more entangled, requiring more classical resources for MPS to simulate with high fidelity. The quantum circuit in question is the circuit of a quantum approximate optimization algorithm that can be used to solve the Max-Cut problem. The main result is that the classical difficulty of simulating such circuits is an increasing function of entanglement per qubit. This is important in the ongoing comparison of traditional simulators and his NISQ processor.
Title: Graphical Quantum Clifford Encoder Compiler from ZX Calculus
Organizations: Massachusetts Institute of Technology; Harvard University
When working with complex systems such as quantum computers, it’s important to work at the right level of abstraction. Traditional compilers map high-level programming languages to Boolean gates, but the concept of compilation is more general, as Peter Shor and co-authors point out in a recent preprint. Considers Clifford arithmetic and compiles them into his ZX calculus diagrams. Compared to traditional schematics, these visualizations emphasize both the propagation of information from inputs to outputs and the entangled structure of outputs.
Title: Fundamentals of Design Tools for Quantum Computing: Arrays, Decision Diagrams, Tensor Networks, and ZX Computation
Organization: Technical University of Munich.Software Competence Center Hagenberg; Johannes Kepler University
As quantum hardware develops, the breadth and depth of circuits will expand, and features such as “in-circuit measurement and reuse” will become more common. Various tools have been developed to insightfully design efficient, effective and useful algorithms. This preprint describes his three tools: decision diagrams, tensor networks, and ZX computation. Clear examples are provided to give the reader an intuition as to how and where they can best be used.
Title: Quantum Machine Learning on Large Datasets with Randomized Measurements
Organization: Imperial College
Some machine learning methods may be more natural to implement on quantum processors than others. Kernel methods work well on classical computers, but they do not produce state-of-the-art results, but they have clear connections to quantum mechanics. The main drawback is that he scales quadratically with the size of the dataset, making it impractical for big data applications. This paper shows that randomized quantum measurements allow quantum kernels to be computed in a time that scales linearly with the size of the dataset. Classification of images of handwritten digits is performed on an IBM quantum computer, implementing error mitigation techniques. Distributing quantum computation across two devices has also been demonstrated. This technique requires traditional post-processing that scales quadratically with the amount of data, but this is not a problem as traditional computing is relatively fast and cheap. This approach thus allows us to explore quantum kernel methods in a more practical way than before.
Title: Limitations of Variational Quantum Algorithms: A Quantum Optimal Transport Approach
Organizations: University of Bologna; University of New Mexico; Technical University of Munich; University of Copenhagen; ENS Lyon
Quantum processors may continue to be very noisy in the near future. So as long as you don’t have too many layers of gates you can definitely do it. In this work, we show that for certain optimization problems, noisy quantum circuits with only a few layers provide no quantum advantage. Is it possible that a noisy quantum device running a long series of gates just happens to give a single output that is more useful than classical optimization algorithms? I’m here.
Title: Gibbs State Expectation Prediction Using Pure Thermal Shadows
The preparation and measurement of quantum states, known as Gibbs states, is a challenging but important task in quantum materials research, optimization and machine learning. Low-energy eigenstates can be implemented using quantum signal processing methods. They found that measuring the outcome state on a randomly chosen basis meant that few measurements were needed to accurately estimate the expected value. They performed a classical simulation to show how this works in a so-called quantum Boltzmann machine. Overall, their approach could be useful for training larger machine learning models.
Title: On-demand electrical control of spin qubits
Organization: University of New South Wales.Dirac; Keio University; Leibniz-Institut für Crystallzüchtung; VITCON Projectconsult GmbH; Simon Fraser University
A key aspect of silicon qubits is their ability to take advantage of CMOS technology that the semiconductor industry has developed over the past decades. To deliver on this promise of scalability, the authors of this paper devised a simple, fast, high-fidelity, all-electrical method to control the spin in silicon. They accelerated the Rabi frequency by a factor of 650 and achieved single-qubit gate durations of 3 ns. Moreover, the ability to switch the electrical interaction on and off could potentially address spins with less noise if it could be done more reliably.
Title: Entangling microwaves with light
Organization: Austrian Institute of Science and Technology; Vienna Center for Quantum Science and Technology
The coherent conversion of single photons between the light and microwave domains makes it possible to network superconducting quantum processors with optical fibers. Quantum transducers could be a key component of the future quantum internet, just as converting between different frequencies of light is important for today’s internet. To this end, the authors of this paper have developed an optical pulsed, ultra-low noise, superconducting, cavity electro-optic modulator. They experimentally demonstrated deterministic quantum entanglement between photons propagating in a millikelvin environment and microwave photons, violating the separability criterion by more than 5 standard deviations.
Title: Manipulation and proof of high-dimensional entanglement in scattering media
Organizations: Sorbonne University; University of Glasgow
High-dimensional entangled states may enhance secure quantum communication protocols and high-performance microscopy. Unfortunately, quantum states are fragile when moving through chaotic and inhomogeneous media. There is turbulence in the atmosphere and random mode mixing in multimode fibers. In this work, we use a classical light beam to infer the details of the scattering medium and a spatial light modulator to prepare spatially entangled photon pairs. By compensating for the perturbations they experience, the researchers managed to untangle the medium’s output. By measuring position-based and momentum-based photons, they prove he has 17-dimensional entanglement, yielding results that violate the Einstein-Podolski-Rosen criterion with 998 standard deviations. Although it is expected to be very difficult to experimentally compensate for perturbations induced by thicker materials, the demonstrated approach is a good starting point for making entangled states more robust in the real world. .
Title: Provably secure quantum randomness extension using uncharacterized homodyne detection
Organization: National University of Singapore. JPMorgan Chase & Company
Random number generators are a key component of many modern computing platforms. Since the measurement of quantum systems is a stochastic process, the development of quantum random number generators (QRNGs) is considered very promising. The goal is to generate a perfectly uniform and unpredictable bit sequence. Security analysis often requires models of QRNG equipment, but this can overestimate the amount of randomness generated, which is disastrous for security applications. A QRNG is described as semi-device independent if researchers can guarantee security with fewer modeling assumptions. In this paper, we build a simple system based on equilibrium homodyne detection. In theory, the author treats this as a black box that does not necessarily produce independently and equally distributed outputs. They took into account finite-size effects and proved that an uncharacterized homodyne detector offers security against side-channel attacks.
Title: Approach to Optimal Entangled Population Measurement on Quantum Computing Platforms
Organization: Australian National University. Friedrich Schiller University of Jena. University of Cambridge; Institute for Experimental Physics, Innsbruck.Fraunhofer Institute for Applied Optics and Precision Engineering IOF; Max Planck School of Photonics. Macquarie University; Amazon Web Services; Quantum Optics and Quantum Information Laboratory, Innsbruck. Alpine Quantum Technology; Nanyang Technological University; Research Organization of Science and Technology (A*STAR)
Quantum states are usually thought to be very delicate and fragile. However, this sensitivity to interaction with the surroundings means that they can be used as excellent probes of environmental fields. By inflicting two quantum states with small Bloch sphere rotations and measuring them together, we can estimate these rotation angles with greater accuracy than can be classically achieved with the same resources. The authors of this paper have achieved this on several quantum computing platforms. Experiments were performed using superconducting, trapped-ion and photonic qubits. Error mitigation was investigated and insightful observations were made regarding the uncertainty relationship. Overall, this work paves the way for a future in which quantum sensors connected to quantum processors can implement enhanced imaging and sensing protocols.
January 31, 2023