926 research outputs found
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
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
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
Quantum-Limited Position Detection and Amplification: A Linear Response Perspective
Using standard linear response relations, we derive the quantum limit on the
sensitivity of a generic linear-response position detector, and the noise
temperature of a generic linear amplifier. Particular emphasis is placed on the
detector's effective temperature and damping effects; the former quantity
directly determines the dimensionless power gain of the detector. Unlike the
approach used in the seminal work of Caves [Phys. Rev. D, 26, 1817 (1982)], the
linear-response approach directly involves the noise properties of the
detector, and allows one to derive simple necessary and sufficient conditions
for reaching the quantum limit. Our results have direct relevance to recent
experiments on nanoelectromechanical systems, and complement recent theoretical
studies of particular mesoscopic position detectors.Comment: 9 pages; minor typos correcte
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