122 results found
Mori T, Zhao H, Mintert F, et al., 2021, Rigorous Bounds on the Heating Rate in Thue-Morse Quasiperiodically and Randomly Driven Quantum Many-Body Systems, PHYSICAL REVIEW LETTERS, Vol: 127, ISSN: 0031-9007
Self CN, Khosla KE, Smith AWR, et al., 2021, Variational quantum algorithm with information sharing, npj Quantum Information, Vol: 7, ISSN: 2056-6387
We introduce an optimisation method for variational quantum algorithms and experimentally demonstrate a 100-fold improvement in efficiency compared to naive implementations. The effectiveness of our approach is shown by obtaining multi-dimensional energy surfaces for small molecules and a spin model. Our method solves related variational problems in parallel by exploiting the global nature of Bayesian optimisation and sharing information between different optimisers. Parallelisation makes our method ideally suited to the next generation of variational problems with many physical degrees of freedom. This addresses a key challenge in scaling-up quantum algorithms towards demonstrating quantum advantage for problems of real-world interest.
The accurate implementation of quantum gates is essential for the realisation of quantum algorithms and digital quantum simulations. This accuracy may be increased on noisy hardware through the variational optimisation of gates, however the experimental realisation of such a protocol is impeded by the large effort required to estimate the fidelity of an implemented gate. With a hierarchy of approximations we find a faithful approximation to the quantum process fidelity that can be estimated experimentally with reduced effort. Its practical use is demonstrated with the optimisation of a three-qubit quantum gate on a commercially available quantum processor.
Petiziol F, Sameti M, Carretta S, et al., 2021, Quantum Simulation of Three-Body Interactions in Weakly Driven Quantum Systems, PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
Sameti M, Lishman J, Mintert F, 2021, Strong-coupling quantum logic of trapped ions, PHYSICAL REVIEW A, Vol: 103, ISSN: 2469-9926
Nyman RA, Dhar HS, Rodrigues JD, et al., 2021, Phase transitions of light in a dye-filled microcavity: observations and simulations, Journal of Physics: Conference Series, Vol: 1919, Pages: 012006-012006, ISSN: 1742-6588
<jats:title>Abstract</jats:title> <jats:p>Photon thermalisation and condensation in dye-filled microcavities is a growing area of scientific interest, at the intersection of photonics, quantum optics and statistical physics. We give here a short introduction to the topic, together with an explanation of some of our more important recent results. A key result across several projects is that we have a model based on a detailed physical description which has been used to accurately describe experimental observations. We present a new open-source package in Python called PyPBEC which implements this model. The aim is to enable the reader to readily simulate and explore the physics of photon condensates themselves, so this article also includes a working example code which can be downloaded from the GitHub repository.</jats:p>
Rodrigues JD, Dhar HS, Walker BT, et al., 2021, Learning the Fuzzy Phases of Small Photonic Condensates, PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
Joseph D, Callison A, Ling C, et al., 2021, Two quantum Ising algorithms for the shortest-vector problem, Physical Review A: Atomic, Molecular and Optical Physics, Vol: 103, Pages: 1-12, ISSN: 1050-2947
Quantum computers are expected to break today's public key cryptography within a few decades. New cryptosystems are being designed and standardized for the postquantum era, and a significant proportion of these rely on the hardness of problems like the shortest-vector problem to a quantum adversary. In this paper we describe two variants of a quantum Ising algorithm to solve this problem. One variant is spatially efficient, requiring only O(Nlog2N) qubits, where N is the lattice dimension, while the other variant is more robust to noise. Analysis of the algorithms' performance on a quantum annealer and in numerical simulations shows that the more qubit-efficient variant will outperform in the long run, while the other variant is more suitable for near-term implementation.
Simsek S, Mintert F, 2021, Quantum control with a multi-dimensional Gaussian quantum invariant, Quantum, Vol: 5, Pages: 1-12, ISSN: 2521-327X
The framework of quantum invariants is an elegant generalization of adiabatic quantum control to control fields that do not need to change slowly. Due to the unavailability of invariants for systems with more than one spatial dimension, the benefits of this framework have not yet been exploited in multi-dimensional systems. We construct a multi-dimensional Gaussian quantum invariant that permits the design of time-dependent potentials that let the ground state of an initial potential evolve towards the ground state of a final potential. The scope of this framework is demonstrated with the task of shuttling an ion around a corner which is a paradigmatic control problem in achieving scalability of trapped ion quantum information technology.
Zhao H, Mintert F, Moessner R, et al., 2021, Random Multipolar Driving: Tunably Slow Heating through Spectral Engineering, PHYSICAL REVIEW LETTERS, Vol: 126, ISSN: 0031-9007
Dhar HS, Rodrigues JD, Walker BT, et al., 2020, Transport and localization of light inside a dye-filled microcavity, Physical Review A: Atomic, Molecular and Optical Physics, Vol: 102, Pages: 1-9, ISSN: 1050-2947
The driven-dissipative nature of light-matter interaction inside a multimode, dye-filled microcavity makes it an ideal system to study nonequilibrium phenomena, such as transport. In this work, we investigate how light is efficiently transported inside such a microcavity, mediated by incoherent absorption and emission processes. In particular, we show that there exist two distinct regimes of transport, viz. conductive and localized, arising from the complex interplay between the thermalizing effect of the dye molecules and the nonequilibrium influence of driving and loss. The propagation of light in the conductive regime occurs when several localized cavity modes undergo dynamical phase transitions to a condensed, or lasing, state. Furthermore, we observe that, while such transport is robust for weak disorder in the cavity potential, strong disorder can lead to localization of light even under good thermalizing conditions. Importantly, the exhibited transport and localization of light is a manifestation of the nonequilibrium dynamics rather than any coherent interference in the system.
Mukherjee R, Xie H, Mintert F, 2020, Bayesian Optimal Control of Greenberger-Horne-Zeilinger States in Rydberg Lattices, PHYSICAL REVIEW LETTERS, Vol: 125, ISSN: 0031-9007
Holmes Z, Mintert F, Anders J, 2020, Gibbs mixing of partially distinguishable photons with a polarising beamsplitter membrane, NEW JOURNAL OF PHYSICS, Vol: 22, ISSN: 1367-2630
Kuroś A, Mukherjee R, Golletz W, et al., 2020, Phase diagram and optimal control for n-tupling discrete time crystal, New Journal of Physics, Vol: 22, Pages: 1-13, ISSN: 1367-2630
Hunter-Gordon M, Szabo Z, Nyman RA, et al., 2020, Quantum simulation of the dephasing Anderson model, Physical Review A: Atomic, Molecular and Optical Physics, Vol: 102, Pages: 022407 – 1-022407 – 4, ISSN: 1050-2947
The interplay of Anderson localization and decoherence results in intricate dynamics that is notoriously difficult to simulate on classical computers. We develop the framework for a quantum simulation of such an open quantum system making use of time-varying randomized gradients, and show that even an implementation with limited experimental resources results in accurate simulations.
Lishman J, Mintert F, 2020, Trapped-ion entangling gates robust against qubit frequency errors, PHYSICAL REVIEW RESEARCH, Vol: 2
Mukherjee R, Sauvage F, Xie H, et al., 2020, Preparation of ordered states in ultra–cold gases using Bayesian optimization, New Journal of Physics, Vol: 22, ISSN: 1367-2630
Ultra-cold atomic gases are unique in terms of the degree of controllability, both for internal and external degrees of freedom. This makes it possible to use them for the study of complex quantum many-body phenomena. However in many scenarios, the prerequisite condition of faithfully preparing a desired quantum state despite decoherence and system imperfections is not always adequately met. To path the way to a specific target state, we explore quantum optimal control framework based on Bayesian optimization. The probabilistic modeling and broad exploration aspects of Bayesian optimization is particularly suitable for quantum experiments where data acquisition can be expensive. Using numerical simulations for the superfluid to Mott- insulator transition for bosons in a lattice as well for the formation of Rydberg crystals as explicit examples, we demonstrate that Bayesian optimization is capable of finding better control solutions with regards to finite and noisy data compared to existing methods of optimal control.
Holmes Z, Anders J, Mintert F, 2020, Enhanced energy transfer to an optomechanical piston from indistinguishable photons, Physical Review Letters, Vol: 124, Pages: 210601-1-210601-6, ISSN: 0031-9007
Thought experiments involving gases and pistons, such as Maxwell’s demon and Gibbs’ mixing, are central to our understanding of thermodynamics. Here we present a quantum thermodynamic thought experiment in which the energy transfer from two photonic gases to a piston membrane grows quadratically with the number of photons for indistinguishable gases, while linearly for distinguishable gases. This signature of Bosonic bunching may be observed in optomechanical experiments, highlighting the potential of these systems for the realization of thermodynamic thought experiments in the quantum realm.
Newman D, Mintert F, Nazir A, 2020, Quantum limit to nonequilibrium heat-engine performance imposed by strong system-reservoir coupling, PHYSICAL REVIEW E, Vol: 101, ISSN: 2470-0045
Rath Y, Mintert F, 2020, Prominent interference peaks in the dephasing Anderson model, PHYSICAL REVIEW RESEARCH, Vol: 2
The concept of quantum many-body scars has recently been put forward as a route to describe weak ergodicity breaking and violation of the eigenstate thermalization hypothesis. We propose a simple setup to generate quantum many-body scars in a doubly modulated Bose-Hubbard system which can be readily implemented in cold atomic gases. The dynamics are shown to be governed by kinetic constraints which appear via density-assisted tunneling in a high-frequency expansion. We find the optimal driving parameters for the kinetically constrained hopping which leads to small isolated subspaces of scared eigenstates. The experimental signatures and the transition to fully thermalizing behavior as a function of driving frequency are analyzed.
Joseph D, Ghionis A, Ling C, et al., 2020, Not-so-adiabatic quantum computation for the shortest vector problem, Physical Review Research, Vol: 2, Pages: 1-13, ISSN: 2643-1564
Since quantum computers are known to break the vast majority of currently used cryptographic protocols, a variety of new protocols are being developed that are conjectured, but not proved, to be safe against quantum attacks. Among the most promising is lattice-based cryptography, where security relies upon problems like the shortest vector problem. We analyze the potential of adiabatic quantum computation for attacks on lattice-based cryptography, and give numerical evidence that even outside the adiabatic regime such methods can facilitate the solution of the shortest vector and similar problems.
Walker BT, Rodrigues JD, Dhar HS, et al., 2020, Non-stationary statistics and formation jitter in transient photon condensation, Publisher: NATURE PUBLISHING GROUP
Marques Rodrigues J, Walker BT, Dhar HS, et al., 2020, Non-stationary statistics and formation jitter in transient photon condensation, Nature Communications, Vol: 11, ISSN: 2041-1723
While equilibrium phase transitions are easily described by order parameters and free-energylandscapes, for their non-stationary counterparts these quantities are usually ill-defined. Here,we probe transient non-equilibrium dynamics of an optically pumped, dye-filled microcavity. Wequench the system to a far-from-equilibrium state and find delayed condensation close to a criticalexcitation energy, a transient equivalent of critical slowing down. Besides number fluctuations nearthe critical excitation energy, we show that transient phase transitions exhibit timing jitter in thecondensate formation. This jitter is a manifestation of the randomness associated with spontaneousemission, showing that condensation is a stochastic, rather than deterministic process. Despite thenon-equilibrium character of this phase transition, we construct an effective free-energy landscapethat describes the formation jitter and allows, in principle, its generalization to a wider class ofprocesses.
Dive B, Koukoulekidis N, Mousafeiris S, et al., 2020, Characterization of multilevel quantum coherence without ideal measurements, Publisher: AMER PHYSICAL SOC
Petiziol F, Arimondo E, Giannelli L, et al., 2020, Optimized three-level quantum transfers based on frequency-modulated optical excitations, SCIENTIFIC REPORTS, Vol: 10, ISSN: 2045-2322
Holmes Z, Mingo EH, Chen CYR, et al., 2020, Quantifying athermality and quantum induced deviations from classical fluctuation relations, Entropy: international and interdisciplinary journal of entropy and information studies, Vol: 22, Pages: 1-29, ISSN: 1099-4300
In recent years a quantum information theoretic framework has emerged forincorporating non-classical phenomena into fluctuation relations. Here weelucidate this framework by exploring deviations from classical fluctuationrelations resulting from the athermality of the initial thermal system andquantum coherence of the system's energy supply. In particular we developCrooks-like equalities for an oscillator system which is prepared either inphoton added or photon subtracted thermal states and derive a Jarzynski-likeequality for average work extraction. We use these equalities to discuss theextent to which adding or subtracting a photon increases the informationalcontent of a state thereby amplifying the suppression of free energy increasingprocess. We go on to derive a Crooks-like equality for an energy supply that isprepared in a pure binomial state, leading to a non-trivial contribution fromenergy and coherence on the resultant irreversibility. We show how the binomialstate equality fits in relation to a previously derived coherent state equalityand offers a richer feature-set.
Callison A, Chancellor N, Mintert F, et al., 2019, Finding spin glass ground states using quantum walks, New Journal of Physics, Vol: 21, Pages: 1-20, ISSN: 1367-2630
Quantum computation using continuous-time evolution under a natural hardware Hamiltonian is a promising near- and mid-term direction toward powerful quantum computing hardware. We investigate the performance of continuous-time quantum walks as a tool for finding spin glass ground states, a problem that serves as a useful model for realistic optimization problems. By performing detailed numerics, we uncover significant ways in which solving spin glass problems differs from applying quantum walks to the search problem. Importantly, unlike for the search problem, parameters such as the hopping rate of the quantum walk do not need to be set precisely for the spin glass ground state problem. Heuristic values of the hopping rate determined from the energy scales in the problem Hamiltonian are sufficient for obtaining a better quantum advantage than for search. We uncover two general mechanisms that provide the quantum advantage: matching the driver Hamiltonian to the encoding in the problem Hamiltonian, and an energy redistribution principle that ensures a quantum walk will find a lower energy state in a short timescale. This makes it practical to use quantum walks for solving hard problems, and opens the door for a range of applications on suitable quantum hardware.
Tranter A, Love P, Mintert F, et al., 2019, Ordering of Trotterization: impact on errors in quantum simulation of electronic structure, Entropy: international and interdisciplinary journal of entropy and information studies, Vol: 21, ISSN: 1099-4300
Trotter–Suzuki decompositions are frequently used in the quantum simulation of quantum chemistry. They transform the evolution operator into a form implementable on a quantum device, while incurring an error—the Trotter error. The Trotter error can be made arbitrarily small by increasing the Trotter number. However, this increases the length of the quantum circuits required, which may be impractical. It is therefore desirable to find methods of reducing the Trotter error through alternate means. The Trotter error is dependent on the order in which individual term unitaries are applied. Due to the factorial growth in the number of possible orderings with respect to the number of terms, finding an optimal strategy for ordering Trotter sequences is difficult. In this paper, we propose three ordering strategies, and assess their impact on the Trotter error incurred. Initially, we exhaustively examine the possible orderings for molecular hydrogen in a STO-3G basis. We demonstrate how the optimal ordering scheme depends on the compatibility graph of the Hamiltonian, and show how it varies with increasing bond length. We then use 44 molecular Hamiltonians to evaluate two strategies based on coloring their incompatibility graphs, while considering the properties of the obtained colorings. We find that the Trotter error for most for systems involving heavy atoms, using a reference magnitude ordering, is less than 1 kcal/mol. Relative to this, the difference between ordering schemes can be substantial, being approximately on the order of millihartrees. The coloring-based ordering schemes are reasonably promising—particularly for systems involving heavy atoms—however further work is required to increase dependence on the magnitude of terms. Finally, we consider ordering strategies based on the norm of the Trotter error operator, including an iterative method for generating the new error operator terms added upon insertion of a term into an ordered Hamil
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