5,137 research outputs found
Optomechanical tailoring of quantum fluctuations
We propose the use of feedback mechanism to control the level of quantum
noise in a radiation field emerging from a pendular Fabry-Perot cavity. It is
based on the possibility to perform quantum nondemolition measurements by means
of optomechanical coupling.Comment: ReVTeX file, 8 pages, 1 Postscript figure. to appear in J. Opt. B:
Quant. Semiclass. Op
Entanglement under restricted operations: Analogy to mixed-state entanglement
We show that the classification of bi-partite pure entangled states when
local quantum operations are restricted yields a structure that is analogous in
many respects to that of mixed-state entanglement. Specifically, we develop
this analogy by restricting operations through local superselection rules, and
show that such exotic phenomena as bound entanglement and activation arise
using pure states in this setting. This analogy aids in resolving several
conceptual puzzles in the study of entanglement under restricted operations. In
particular, we demonstrate that several types of quantum optical states that
possess confusing entanglement properties are analogous to bound entangled
states. Also, the classification of pure-state entanglement under restricted
operations can be much simpler than for mixed-state entanglement. For instance,
in the case of local Abelian superselection rules all questions concerning
distillability can be resolved.Comment: 10 pages, 2 figures; published versio
There is no unmet requirement of optical coherence for continuous-variable quantum teleportation
It has been argued [T. Rudolph and B.C. Sanders, Phys. Rev. Lett. 87, 077903
(2001)] that continuous-variable quantum teleportation at optical frequencies
has not been achieved because the source used (a laser) was not `truly
coherent'. Here I show that `true coherence' is always illusory, as the concept
of absolute time on a scale beyond direct human experience is meaningless. A
laser is as good a clock as any other, even in principle, and this objection to
teleportation experiments is baseless.Comment: 6 pages, no figures, no equations, to be published in Journal of
Modern Optics. This is a long version of quant-ph/0104004. I have not
replaced that paper with this one because some authors have referenced that
one approvingly who may feel differently about doing so to this versio
Heterodyne and adaptive phase measurements on states of fixed mean photon number
The standard technique for measuring the phase of a single mode field is
heterodyne detection. Such a measurement may have an uncertainty far above the
intrinsic quantum phase uncertainty of the state. Recently it has been shown
[H. M. Wiseman and R. B. Killip, Phys. Rev. A 57, 2169 (1998)] that an adaptive
technique introduces far less excess noise. Here we quantify this difference by
an exact numerical calculation of the minimum measured phase variance for the
various schemes, optimized over states with a fixed mean photon number. We also
analytically derive the asymptotics for these variances. For the case of
heterodyne detection our results disagree with the power law claimed by
D'Ariano and Paris [Phys. Rev. A 49, 3022 (1994)].Comment: 9 pages, 2 figures, minor changes from journal versio
Optimal states and almost optimal adaptive measurements for quantum interferometry
We derive the optimal N-photon two-mode input state for obtaining an estimate
\phi of the phase difference between two arms of an interferometer. For an
optimal measurement [B. C. Sanders and G. J. Milburn, Phys. Rev. Lett. 75, 2944
(1995)], it yields a variance (\Delta \phi)^2 \simeq \pi^2/N^2, compared to
O(N^{-1}) or O(N^{-1/2}) for states considered by previous authors. Such a
measurement cannot be realized by counting photons in the interferometer
outputs. However, we introduce an adaptive measurement scheme that can be thus
realized, and show that it yields a variance in \phi very close to that from an
optimal measurement.Comment: 4 pages, 4 figures, journal versio
Reconsidering Rapid Qubit Purification by Feedback
This paper reconsiders the claimed rapidity of a scheme for the purification
of the quantum state of a qubit, proposed recently in Jacobs 2003 Phys. Rev.
A67 030301(R). The qubit starts in a completely mixed state, and information is
obtained by a continuous measurement. Jacobs' rapid purification protocol uses
Hamiltonian feedback control to maximise the average purity of the qubit for a
given time, with a factor of two increase in the purification rate over the
no-feedback protocol. However, by re-examining the latter approach, we show
that it mininises the average time taken for a qubit to reach a given purity.
In fact, the average time taken for the no-feedback protocol beats that for
Jacobs' protocol by a factor of two. We discuss how this is compatible with
Jacobs' result, and the usefulness of the different approaches.Comment: 11 pages, 3 figures. Final version, accepted for publication in New
J. Phy
Quantum error correction for continuously detected errors
We show that quantum feedback control can be used as a quantum error
correction process for errors induced by weak continuous measurement. In
particular, when the error model is restricted to one, perfectly measured,
error channel per physical qubit, quantum feedback can act to perfectly protect
a stabilizer codespace. Using the stabilizer formalism we derive an explicit
scheme, involving feedback and an additional constant Hamiltonian, to protect
an ()-qubit logical state encoded in physical qubits. This works for
both Poisson (jump) and white-noise (diffusion) measurement processes. In
addition, universal quantum computation is possible in this scheme. As an
example, we show that detected-spontaneous emission error correction with a
driving Hamiltonian can greatly reduce the amount of redundancy required to
protect a state from that which has been previously postulated [e.g., Alber
\emph{et al.}, Phys. Rev. Lett. 86, 4402 (2001)].Comment: 11 pages, 1 figure; minor correction
Adaptive single-shot phase measurements: The full quantum theory
The phase of a single-mode field can be measured in a single-shot measurement
by interfering the field with an effectively classical local oscillator of
known phase. The standard technique is to have the local oscillator detuned
from the system (heterodyne detection) so that it is sometimes in phase and
sometimes in quadrature with the system over the course of the measurement.
This enables both quadratures of the system to be measured, from which the
phase can be estimated. One of us [H.M. Wiseman, Phys. Rev. Lett. 75, 4587
(1995)] has shown recently that it is possible to make a much better estimate
of the phase by using an adaptive technique in which a resonant local
oscillator has its phase adjusted by a feedback loop during the single-shot
measurement. In Ref.~[H.M. Wiseman and R.B. Killip, Phys. Rev. A 56, 944] we
presented a semiclassical analysis of a particular adaptive scheme, which
yielded asymptotic results for the phase variance of strong fields. In this
paper we present an exact quantum mechanical treatment. This is necessary for
calculating the phase variance for fields with small photon numbers, and also
for considering figures of merit other than the phase variance. Our results
show that an adaptive scheme is always superior to heterodyne detection as far
as the variance is concerned. However the tails of the probability distribution
are surprisingly high for this adaptive measurement, so that it does not always
result in a smaller probability of error in phase-based optical communication.Comment: 17 pages, LaTeX, 8 figures (concatenated), Submitted to Phys. Rev.
In-loop squeezing is real squeezing to an in-loop atom
Electro-optical feedback can produce an in-loop photocurrent with arbitrarily
low noise. This is not regarded as evidence of `real' squeezing because
squeezed light cannot be extracted from the loop using a linear beam splitter.
Here I show that illuminating an atom (which is a nonlinear optical element)
with `in-loop' squeezed light causes line-narrowing of one quadrature of the
atom's fluorescence. This has long been regarded as an effect which can only be
produced by squeezing. Experiments on atoms using in-loop squeezing should be
much easier than those with conventional sources of squeezed light.Comment: 4 pages, 2 figures, submitted to PR
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