3,306 research outputs found
Detection of short-term changes in vegetation cover by use of LANDSAT imagery
The author has identified the following significant results. By using a constant band 6 to band 5 radiance ratio of 1.25, the changing pattern of areas of relatively dense vegetation cover was detected for the semiarid region in the vicinity of Tucson, Arizona. Electronically produced binary thematic masks were used to map areas with dense vegetation. The foliar cover threshold represented by the ratio was not accurately determined but field measurements show that the threshold lies in the range of 10 to 25 percent foliage cover. Montana evergreen forests with constant dense cover were correctly shown to exceed the threshold on all dates. The summer active grassland exceeded the threshold in the summer unless rainfall was insufficient. Desert areas exceeded the threshold during the spring of 1973 following heavy rains; the same areas during the rainless spring of 1974 did not exceed threshold. Irrigated fields, parks, golf courses, and riparian communities were among the habitats most frequently surpassing the threshold
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
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.
On quantum error-correction by classical feedback in discrete time
We consider the problem of correcting the errors incurred from sending
quantum information through a noisy quantum environment by using classical
information obtained from a measurement on the environment. For discrete time
Markovian evolutions, in the case of fixed measurement on the environment, we
give criteria for quantum information to be perfectly corrigible and
characterize the related feedback. Then we analyze the case when perfect
correction is not possible and, in the qubit case, we find optimal feedback
maximizing the channel fidelity.Comment: 11 pages, 1 figure, revtex
State and dynamical parameter estimation for open quantum systems
Following the evolution of an open quantum system requires full knowledge of
its dynamics. In this paper we consider open quantum systems for which the
Hamiltonian is ``uncertain''. In particular, we treat in detail a simple system
similar to that considered by Mabuchi [Quant. Semiclass. Opt. 8, 1103 (1996)]:
a radiatively damped atom driven by an unknown Rabi frequency (as
would occur for an atom at an unknown point in a standing light wave). By
measuring the environment of the system, knowledge about the system state, and
about the uncertain dynamical parameter, can be acquired. We find that these
two sorts of knowledge acquisition (quantified by the posterior distribution
for , and the conditional purity of the system, respectively) are quite
distinct processes, which are not strongly correlated. Also, the quality and
quantity of knowledge gain depend strongly on the type of monitoring scheme. We
compare five different detection schemes (direct, adaptive, homodyne of the
quadrature, homodyne of the quadrature, and heterodyne) using four
different measures of the knowledge gain (Shannon information about ,
variance in , long-time system purity, and short-time system purity).Comment: 14 pages, 18 figure
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
Simulation of interaction Hamiltonians by quantum feedback: a comment on the dynamics of information exchange between coupled systems
Since quantum feedback is based on classically accessible measurement
results, it can provide fundamental insights into the dynamics of quantum
systems by making available classical information on the evolution of system
properties and on the conditional forces acting on the system. In this paper,
the feedback-induced interaction dynamics between a pair of quantum systems is
analyzed. It is pointed out that any interaction Hamiltonian can be simulated
by local feedback if the levels of decoherence are sufficiently high. The
boundary between genuine entanglement generating quantum interactions and
non-entangling classical interactions is identified and the nature of the
information exchange between two quantum systems during an interaction is
discussed.Comment: 14 pages, 4 figures; invited paper for the special issue of J. Opt. B
on quantum contro
Quantum optical waveform conversion
Currently proposed architectures for long-distance quantum communication rely
on networks of quantum processors connected by optical communications channels
[1,2]. The key resource for such networks is the entanglement of matter-based
quantum systems with quantum optical fields for information transmission. The
optical interaction bandwidth of these material systems is a tiny fraction of
that available for optical communication, and the temporal shape of the quantum
optical output pulse is often poorly suited for long-distance transmission.
Here we demonstrate that nonlinear mixing of a quantum light pulse with a
spectrally tailored classical field can compress the quantum pulse by more than
a factor of 100 and flexibly reshape its temporal waveform, while preserving
all quantum properties, including entanglement. Waveform conversion can be used
with heralded arrays of quantum light emitters to enable quantum communication
at the full data rate of optical telecommunications.Comment: submitte
Rapid purification of quantum systems by measuring in a feedback-controlled unbiased basis
Rapid-purification by feedback --- specifically, reducing the mean impurity
faster than by measurement alone --- can be achieved by making the eigenbasis
of the density matrix to be unbiased relative to the measurement basis. Here we
further examine the protocol introduced by Combes and Jacobs [Phys.Rev.Lett.
{\bf 96}, 010504 (2006)] involving continuous measurement of the observable
for a -dimensional system. We rigorously re-derive the lower bound
on the achievable speed-up factor, and also an upper bound, namely
, for all feedback protocols that use measurements in unbiased bases.
Finally we extend our results to independent measurements on a register of
qubits, and derive an upper bound on the achievable speed-up factor that
scales linearly with .Comment: v2: published versio
Optomechanical Cooling of a Macroscopic Oscillator by Homodyne Feedback
We propose a simple optomechanical model in which a mechanical oscillator
quadrature could be "cooled" well below its equilibrium temperature by applying
a suitable feedback to drive the orthogonal quadrature by means of the homodyne
current of the radiation field used to probe its position.Comment: 9 pages, RevTeX, Figures available from authors, to appear in Phys.
Rev. Let
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