Optomechanical dual-beam backaction-evading measurement beyond the rotating-wave approximation

We present the exact analytical solution of the explicitly time-periodic quantum Langevin equation describing the dual-beam backaction-evading measurement of a single mechanical oscillator quadrature due to V. B. Braginsky, Y. I. Vorontsov, and K. S. Thorne [Science 209, 547 (1980)] beyond the commonly used rotating-wave approximation. We show that counterrotating terms lead to extra sidebands in the optical and mechanical spectra and to a modification of the main peak. Physically, the backaction of the measurement is due to periodic coupling of the mechanical resonator to a light-field quadrature that only contains cavity-filtered shot noise. Since this fact is independent of other degrees of freedom the resonator might be coupled to, our solution can be generalized, including to dissipatively or parametrically squeezed oscillators, as well as recent two-mode backaction-evading measurements.Royal Society (University Research Fellowship), Winton Programme for the Physics of Sustainabilit

Floquet approach to bichromatically driven cavity-optomechanical systems

We develop a Floquet approach to solve time-periodic quantum Langevin equations in steady state. We show that two-time correlation functions of system operators can be expanded in a Fourier series and that a generalized Wiener-Khinchin theorem relates the Fourier transform of their zeroth Fourier component to the measured spectrum. We apply our framework to bichromatically driven cavity optomechanical systems, a setting in which mechanical oscillators have recently been prepared in quantum-squeezed states. Our method provides an intuitive way to calculate the power spectral densities for time-periodic quantum Langevin equations in arbitrary rotating frames.A.N. holds a University Research Fellowship from the Royal Society and acknowledges additional support from the Winton Programme for the Physics of Sustainability. D.M. acknowledges support by the UK Engineering and Physical Sciences Research Council (EPSRC) under Grant No. EP/M506485/1.This is the author accepted manuscript. The final version is available from the American Physical Society via http://dx.doi.org/10.1103/PhysRevA.94.02380

Current rectification in a double quantum dot through fermionic reservoir engineering

Reservoir engineering is a powerful tool for the robust generation of quantum states or transport properties. Using both a weak-coupling quantum master equation and the exact solution, we show that directional transport of electrons through a double quantum dot can be achieved through an appropriately designed electronic environment. Directionality is attained through the interference of coherent and dissipative coupling. The relative phase is tuned with an external magnetic field, such that directionality can be reversed, as well as turned on and off dynamically. Our work introduces fermionic reservoir engineering, paving the way to a new class of nanoelectronic devices

Preprint arXiv: 2211.16439 Submitted on 29 Nov 2022

We experimentally assess the suitability of transmon qubits with fixedfrequencies and fixed interactions for the realization of analogue quantumsimulations of spin systems. We test a set of necessary criteria for this goalon a commercial quantum processor using full quantum process tomography andmore efficient Hamiltonian tomography. Significant single qubit errors at lowamplitudes are identified as a limiting factor preventing the realization ofanalogue simulations on currently available devices. We additionally findspurious dynamics in the absence of drive pulses, which we identify withcoherent coupling between the qubit and a low dimensional environment. Withmoderate improvements, analogue simulation of a rich family of time-dependentmany-body spin Hamiltonians may be possible

Preprint arXiv:2212.07789 Submitted on 15 Dec 2022

Intermediate-scale quantum devices are becoming more reliable, and may soonbe harnessed to solve useful computational tasks. At the same time, commonclassical methods used to verify their computational output become intractabledue to a prohibitive scaling of required resources with system size. Inspiredby recent experimental progress, here we describe and analyze efficientcross-platform verification protocols for quantum states and show how these canbe used to verify computations. We focus on the pair-wise comparison betweendistant nodes of a quantum network, identify the most promising protocols andthen discuss how they can be implemented in laboratory settings. As a proof ofprinciple, we implement basic versions of these schemes on available quantumprocessors

Generation of photonic tensor network states with Circuit QED

We propose a circuit QED platform and protocol to deterministically generate microwave photonic tensor network states. We first show that using a microwave cavity as ancilla and a transmon qubit as emitter is a favorable platform to produce photonic matrix-product states. The ancilla cavity combines a large controllable Hilbert space with a long coherence time, which we predict translates into a high number of entangled photons and states with a high bond dimension. Going beyond this paradigm, we then consider a natural generalization of this platform, in which several cavity--qubit pairs are coupled to form a chain. The photonic states thus produced feature a two-dimensional entanglement structure and are readily interpreted as $\textit{radial plaquette}$ projected entangled pair states, which include many paradigmatic states, such as the broad class of isometric tensor network states, graph states, string-net states

Convergence Guarantees for Discrete Mode Approximations to Non-Markovian Quantum Baths

Non-Markovian effects are important in modeling the behavior of open quantum systems arising in solid-state physics, quantum optics as well as in study of biological and chemical systems. The non-Markovian environment is often approximated by discrete bosonic modes, thus mapping it to a Lindbladian or Hamiltonian simulation problem. While systematic constructions of such modes have been previously proposed, the resulting approximation lacks rigorous and general convergence guarantees. In this letter, we show that under some physically motivated assumptions on the system-environment interaction, the finite-time dynamics of the non-Markovian open quantum system computed with a sufficiently large number of modes is guaranteed to converge to the true result. Furthermore, we show that this approximation error typically falls off polynomially with the number of modes. Our results lend rigor to classical and quantum algorithms for approximating non-Markovian dynamics

Sequential generation of projected entangled-pair states

We introduce plaquette projected entangled-pair states, a class of states in a lattice that can be generated by applying sequential unitaries acting on plaquettes of overlapping regions. They satisfy area-law entanglement, possess long-range correlations, and naturally generalize other relevant classes of tensor network states. We identify a subclass that can be more efficiently prepared in a radial fashion and that contains the family of isometric tensor network states. We also show how such subclass can be efficiently prepared using an array of photon sources

Supersymmetric Free Fermions and Bosons: Locality, Symmetry and Topology

Supersymmetry, originally proposed in particle physics, refers to a dual relation that connects fermionic and bosonic degrees of freedom in a system. Recently, there has been considerable interest in applying the idea of supersymmetry to topological phases, motivated by the attempt to gain insights from the fermion side into the boson side and vice versa. We present a systematic study of this construction when applied to band topology in noninteracting systems. First, on top of the conventional ten-fold way, we find that topological insulators and superconductors are divided into three classes depending on whether the supercharge can be local and symmetric, must break a symmetry to preserve locality, or needs to break locality. Second, we resolve the apparent paradox between the nontriviality of free fermions and the triviality of free bosons by noting that the topological information is encoded in the identification map. We also discuss how to understand a recently revealed supersymmetric entanglement duality in this context. These findings are illustrated by prototypical examples. Our work sheds new light on band topology from the perspective of supersymmetry