1,485 research outputs found
Cool for Cats
The iconic Schr\"odinger's cat state describes a system that may be in a
superposition of two macroscopically distinct states, for example two clearly
separated oscillator coherent states. Quite apart from their role in
understanding the quantum classical boundary, such states have been suggested
as offering a quantum advantage for quantum metrology, quantum communication
and quantum computation. As is well known these applications have to face the
difficulty that the irreversible interaction with an environment causes the
superposition to rapidly evolve to a mixture of the component states in the
case that the environment is not monitored. Here we show that by engineering
the interaction with the environment there exists a large class of systems that
can evolve irreversibly to a cat state. To be precise we show that it is
possible to engineer an irreversible process so that the steady state is close
to a pure Schr\"odinger's cat state by using double well systems and an
environment comprising two-photon (or phonon) absorbers. We also show that it
should be possible to prolong the lifetime of a Schr\"odinger's cat state
exposed to the destructive effects of a conventional single-photon decohering
environment. Our protocol should make it easier to prepare and maintain
Schr\"odinger cat states which would be useful in applications of quantum
metrology and information processing as well as being of interest to those
probing the quantum to classical transition.Comment: 10 pages, 7 figures. Significantly updated version with supplementary
informatio
Quantum Estimation of Parameters of Classical Spacetimes
We describe a quantum limit to measurement of classical spacetimes.
Specifically, we formulate a quantum Cramer-Rao lower bound for estimating the
single parameter in any one-parameter family of spacetime metrics. We employ
the locally covariant formulation of quantum field theory in curved spacetime,
which allows for a manifestly background-independent derivation. The result is
an uncertainty relation that applies to all globally hyperbolic spacetimes.
Among other examples, we apply our method to detection of gravitational waves
using the electromagnetic field as a probe, as in laser-interferometric
gravitational-wave detectors. Other applications are discussed, from
terrestrial gravimetry to cosmology.Comment: 23 pages. This article supersedes arXiv:1108.522
Frequency up- and down-conversions in two-mode cavity quantum electrodynamics
In this letter we present a scheme for the implementation of frequency up-
and down-conversion operations in two-mode cavity quantum electrodynamics
(QED). This protocol for engineering bilinear two-mode interactions could
enlarge perspectives for quantum information manipulation and also be employed
for fundamental tests of quantum theory in cavity QED. As an application we
show how to generate a two-mode squeezed state in cavity QED (the original
entangled state of Einstein-Podolsky-Rosen)
Quantum interface between an electrical circuit and a single atom
We show how to bridge the divide between atomic systems and electronic
devices by engineering a coupling between the motion of a single ion and the
quantized electric field of a resonant circuit. Our method can be used to
couple the internal state of an ion to the quantized circuit with the same
speed as the internal-state coupling between two ions. All the well-known
quantum information protocols linking ion internal and motional states can be
converted to protocols between circuit photons and ion internal states. Our
results enable quantum interfaces between solid state qubits, atomic qubits,
and light, and lay the groundwork for a direct quantum connection between
electrical and atomic metrology standards.Comment: Supplemental material available on reques
Teleportation with a uniformly accelerated partner
In this work, we give a description of the process of teleportation between
Alice in an inertial frame, and Rob who is in uniform acceleration with respect
to Alice. The fidelity of the teleportation is reduced due to Unruh radiation
in Rob's frame. In so far as teleportation is a measure of entanglement, our
results suggest that quantum entanglement is degraded in non inertial frames.Comment: 7 pages with 4 figures (in revtex4
Quantum limits to all-optical phase shifts in a Kerr nonlinear medium
We consider two copropagating fields in a nonlinear Kerr medium, each with a particular phase and intensity. The Kerr medium possesses an intensity-dependent refractive index and the phase shift of each field thus depends on the intensities of the fields. Classically it is possible to induce an arbitrary phase shift of one field (the signal field) by either increasing the intensity of the other field (the control field) or by increasing the interaction legnth. We show that if the intensity of the control field is low, the phase shift on the signal is limited by the discrete nature of the photon-number distribution in the control field and cannot be increased simply by increasing the interaction length. In general the maximum phase shift of the signal field is φ if the control field possesses φ photons. This limit arises as a consequence of quantum recurrence effects
Quantum nondemolition measurements in optical cavities
We analyze schemes for performing quantum nondemolition (QND) measurements in optical cavities. We consider three schemes: (1) measurement of a quadrature phase amplitude using a parametric process, (2) measurement of a quadrature phase amplitude using the optical Kerr effect in a nonlinear fiber, and (3) measurement of the photon number also using the Kerr effect in a fiber. We show that in the second scheme an enhancement of the QND effect may be obtained by making the cavity finesse for the signal larger than that for the probe
Second Law of Thermodynamics with Discrete Quantum Feedback Control
A new thermodynamic inequality is derived which leads to the maximum work
that can be extracted from multi-heat baths with the assistance of discrete
quantum feedback control. The maximum work is determined by the free-energy
difference and a generalized mutual information content between the
thermodynamic system and the feedback controller. This maximum work can exceed
that in conventional thermodynamics and, in the case of a heat cycle with two
heat baths, the heat efficiency can be greater than that of the Carnot cycle.
The consistency of our results with the second law of thermodynamics is ensured
by the fact that work is needed for information processing of the feedback
controller
Quantum Connectivity of Space-Time and Gravitationally Induced Decorrelation of Entanglement
We discuss an alternative formulation of the problem of quantum optical
fields in a curved space-time using localized operators. We contrast the new
formulation with the standard approach and find observable differences for
entangled states. We propose an experiment in which an entangled pair of
optical pulses are propagated through non-uniform gravitational fields and find
that the new formulation predicts de-correlation of the optical entanglement
under experimentally realistic conditions
Scalable quantum field simulations of conditioned systems
We demonstrate a technique for performing stochastic simulations of
conditional master equations. The method is scalable for many quantum-field
problems and therefore allows first-principles simulations of multimode bosonic
fields undergoing continuous measurement, such as those controlled by
measurement-based feedback. As examples, we demonstrate a 53-fold speed
increase for the simulation of the feedback cooling of a single trapped
particle, and the feedback cooling of a quantum field with 32 modes, which
would be impractical using previous brute force methods.Comment: 5 pages, 2 figure
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