Dynamical bistability in the driven circuit QED

We show that the nonlinear response of a driven circuit quantum electrodynamics setup displays antiresonant multiphoton transitions, as recently observed in a transmon qubit device. By including photon leaking, we explain the lineshape by a perturbative and a semiclassical analysis. We derive a bistable semiclassical quasienergy surface whose lowest quasienergy eigenstate is squeezed, allowing for a squeezing-dependent local effective temperature. We study the escape dynamics out of the metastable state and find signatures of dynamical tunneling, similar as for the quantum Duffing oscillator.Comment: submitted to PR

Topological Phases of Sound and Light

Topological states of matter are particularly robust, since they exploit global features insensitive to local perturbations. In this work, we describe how to create a Chern insulator of phonons in the solid state. The proposed implementation is based on a simple setting, a dielectric slab with a suitable pattern of holes. Its topological properties can be wholly tuned in-situ by adjusting the amplitude and frequency of a driving laser that controls the optomechanical interaction between light and sound. The resulting chiral, topologically protected phonon transport along the edges can be probed completely optically. Moreover, we identify a regime of strong mixing between photon and phonon excitations, which gives rise to a large set of different topological phases. This would be an example of a Chern insulator produced from the interaction between two physically very different particle species, photons and phonons

Optomechanical position detection enhanced by de-amplification using intracavity squeezing

It has been predicted and experimentally demonstrated that by injecting squeezed light into an optomechanical device it is possible to enhance the precision of a position measurement. Here, we present a fundamentally different approach where the squeezing is created directly inside the cavity by a nonlinear medium. Counterintuitively, the enhancement of the signal to noise ratio works by de-amplifying precisely the quadrature that is sensitive to the mechanical motion without losing quantum information. This enhancement works for systems with a weak optomechanical coupling and/or strong mechanical damping. This could allow for larger mechanical bandwidth of quantum limited detectors based on optomechanical devices. Our approach can be straightforwardly extended to Quantum Non Demolition (QND) qubit detection.Comment: references added, slight change

Optomechanical creation of magnetic fields for photons on a lattice

We propose using the optomechanical interaction to create artificial magnetic fields for photons on a lattice. The ingredients required are an optomechanical crystal, i.e. a piece of dielectric with the right pattern of holes, and two laser beams with the right pattern of phases. One of the two proposed schemes is based on optomechanical modulation of the links between optical modes, while the other is an lattice extension of optomechanical wavelength-conversion setups. We illustrate the resulting optical spectrum, photon transport in the presence of an artificial Lorentz force, edge states, and the photonic Aharonov-Bohm effect. Moreover, wWe also briefly describe the gauge fields acting on the synthetic dimension related to the phonon/photon degree of freedom. These can be generated using a single laser beam impinging on an optomechanical array

Confinement-induced resonances for a two-component ultracold atom gas in arbitrary quasi-one-dimensional traps

We solve the two-particle s-wave scattering problem for ultracold atom gases confined in arbitrary quasi-one-dimensional trapping potentials, allowing for two different atom species. As a consequence, the center-of-mass and relative degrees of freedom do not factorize. We derive bound-state solutions and obtain the general scattering solution, which exhibits several resonances in the 1D scattering length induced by the confinement. We apply our formalism to two experimentally relevant cases: (i) interspecies scattering in a two-species mixture, and (ii) the two-body problem for a single species in a non-parabolic trap.Comment: 22 pages, 3 figure

Rapid Exploration of Topological Band Structures using Deep Learning

The design of periodic nanostructures allows to tailor the transport of photons, phonons, and matter waves for specific applications. Recent years have seen a further expansion of this field by engineering topological properties. However, what is missing currently are efficient ways to rapidly explore and optimize band structures and to classify their topological characteristics, for arbitrary unit cell geometries. In this work, we show how deep learning can address this challenge. We introduce an approach where a neural network first maps the geometry to a tight-binding model. This allows us to exploit any underlying space group and predict the symmetries of Bloch waves. We demonstrate how that helps to rapidly categorize a large set of geometries in terms of their band representations, identifying designs for fragile topologies. Engineering of domain walls and optimization are also accelerated by orders of magnitude. The approach is general enough to permit future applications to the geometry discovery in other classes of materials (e.g. active and nonlinear metamaterials)

Tunneling-induced fractal transmission in Valley Hall waveguides

The Valley Hall effect provides a popular route to engineer robust waveguides for bosonic excitations such a photons and phonons. The almost complete absence of backscattering in many experiments has its theoretical underpinning in a smooth-envelope approximation that neglects large momentum transfer and is accurate only for small bulk band gaps and/or smooth domain walls. For larger bulk band gaps and hard domain walls backscattering is expected to become significant. Here, we show that in this experimentally relevant regime, the reflection of a wave at a sharp corner becomes highly sensitive on the orientation of the outgoing waveguide relative to the underlying lattice. Enhanced backscattering can be understood as being triggered by resonant tunneling transitions in quasimomentum space. Tracking the resonant tunneling energies as a function of the waveguide orientation reveals a self-repeating fractal pattern that is also imprinted in the density of states and the backscattering rate at a sharp corner

Phase Space Crystal Vibrations: Chiral Edge States with Preserved Time-reversal Symmetry

Chiral transport along edge channels in Chern insulators represents the most robust version of topological transport, but it usually requires breaking of the physical time-reversal symmetry. In this work, we introduce a different mechanism that foregoes this requirement, based on the combination of the symplectic geometry of phase space and interactions. Starting from a honeycomb phase-space crystal of atoms, which can be generated by periodic driving of a one-dimensional interacting quantum gas, we show that the resulting vibrational lattice waves have topological properties. Our work provides a new platform to study topological many-body physics in dynamical systems

Gradient Ascent Pulse Engineering with Feedback

Efficient approaches to quantum control and feedback are essential for quantum technologies, from sensing to quantum computation. Pure control tasks have been successfully solved using optimization techniques, including methods like gradient-ascent pulse engineering (GRAPE) , relying on a differentiable model of the quantum dynamics. For feedback tasks, such methods are not directly applicable, since the aim is to discover strategies conditioned on measurement outcomes. There, model-free reinforcement learning (RL) has recently proven a powerful new ansatz. What is missing is a way to combine the best of both approaches for scenarios that go beyond weak measurements. In this work, we introduce feedback-GRAPE, which borrows concepts from model-free RL to incorporate the response to strong stochastic (discrete or continuous) measurements, while still performing direct gradient ascent through the quantum dynamics. We illustrate its power on a Jaynes-Cummings model with feedback, where it yields interpretable feedback strategies for state preparation and stabilization in the presence of noise. This approach could be employed for discovering strategies in a wide range of feedback tasks, from calibration of multi-qubit devices to linear-optics quantum computation strategies, quantum-enhanced sensing with adaptive measurements, and quantum error correction