Nonreciprocal Photon Transmission and Amplification via Reservoir Engineering

We discuss a general method for constructing nonreciprocal, cavity-based photonic devices, based on matching a given coherent interaction with its corresponding dissipative counterpart; our method generalizes the basic structure used in the theory of cascaded quantum systems, and can render an extremely wide class of interactions directional. In contrast to standard interference-based schemes, our approach allows directional behavior over a wide bandwidth. We show how it can be used to devise isolators and directional, quantum-limited amplifiers. We discuss in detail how this general method allows the construction of a directional, noise-free phase-sensitive amplifier that is not limited by any fundamental gain-bandwidth constraint. Our approach is particularly well-suited to implementations using superconducting microwave circuits and optomechanical systems.Comment: 15 pages, 6 figure

Accelerated adiabatic quantum gates: optimizing speed versus robustness

We develop new protocols for high-fidelity single qubit gates that exploit and extend theoretical ideas for accelerated adiabatic evolution. Our protocols are compatible with qubit architectures with highly isolated logical states, where traditional approaches are problematic; a prime example are superconducting fluxonium qubits. By using an accelerated adiabatic protocol we can enforce the desired adiabatic evolution while having gate times that are comparable to the inverse adiabatic energy gap (a scale that is ultimately set by the amount of power used in the control pulses). By modelling the effects of decoherence, we explore the tradeoff between speed and robustness that is inherent to shortcuts-to-adiabaticity approaches

Entanglement Dynamics in a Dispersively Coupled Qubit-Oscillator System

We study entanglement dynamics in a system consisting of a qubit dispersively coupled to a finite-temperature, dissipative, driven oscillator. We show that there are two generic ways to generate entanglement: one can entangle the qubit either with the phase or the amplitude of the oscillator's motion. Using an exact solution of the relevant quantum master equation, we study the robustness of both these kinds of entanglement against the effects of dissipation and temperature; in the limit of zero temperature (but finite damping), a simple analytic expression is derived for the logarithmic negativity. We also discuss how the generated entanglement may be detected via dephasing revivals, being mindful that revivals can occur even in the absence of any useful entanglement. Our results have relevance to quantum electromechanics, as well as to circuit QED systems.Comment: 5 pages, 5 figure

Full counting statistics and conditional evolution in a nanoelectromechanical system

We study theoretically the full distribution of transferred charge in a tunnel junction (or quantum point contact) coupled to a nanomechanical oscillator, as well as the conditional evolution of the oscillator. Even if the oscillator is very weakly coupled to the tunnel junction, it can strongly affect the tunneling statistics and lead to a highly non-Gaussian distribution. Conversely, given a particular measurement history of the current, the oscillator energy distribution may be localized and highly non-thermal. We also discuss non-Gaussian correlations between the oscillator motion and tunneling electrons; these show that the tunneling back-action cannot be fully described as an effective thermal bath coupled to the oscillator.Comment: 7 pages; figure added; typos correcte