216 research outputs found
Quantum Trajectories for Realistic Detection
Quantum trajectories describe the stochastic evolution of an open quantum
system conditioned on continuous monitoring of its output, such as by an ideal
photodetector. Here we derive (non-Markovian) quantum trajectories for
realistic photodetection, including the effects of efficiency, dead time,
bandwidth, electronic noise, and dark counts. We apply our theory to a
realistic cavity QED scenario and investigate the impact of such detector
imperfections on the conditional evolution of the system state. A practical
theory of quantum trajectories with realistic detection will be essential for
experimental and technological applications of quantum feedback in many areas.Comment: 5 pages, 4 figures (3 .eps included, 1 jpeg as an additional file).
To be published in Phys. Rev.
Dynamical parameter estimation using realistic photodetection
We investigate the effect of imperfections in realistic detectors upon the
problem of quantum state and parameter estimation by continuous monitoring of
an open quantum system. Specifically, we have reexamined the system of a
two-level atom with an unknown Rabi frequency introduced by Gambetta and
Wiseman [Phys. Rev. A 64, 042105 (2001)]. We consider only direct
photodetection and use the realistic quantum trajectory theory reported by
Warszawski, Wiseman, and Mabuchi [Phys. Rev. A 65, 023802 (2002)]. The most
significant effect comes from a finite bandwidth, corresponding to an
uncertainty in the response time of the photodiode. Unless the bandwidth is
significantly greater than the Rabi frequency, the observer's ability to obtain
information about the unknown Rabi frequency, and about the state of the atom,
is severely compromised. This result has implications for quantum control in
the presence of unknown parameters for realistic detectors, and even for ideal
detectors, as it implies that most of the information in the measurement record
is contained in the precise timing of the detections.Comment: 8 pages, 6 figure
Effect of frequency mismatched photons in quantum information processing
Many promising schemes for quantum information processing (QIP) rely on
few-photon interference effects. In these proposals, the photons are treated as
being indistinguishable particles. However, single photon sources are typically
subject to variation from device to device. Thus the photons emitted from
different sources will not be perfectly identical, and there will be some
variation in their frequencies. Here, we analyse the effect of this frequency
mismatch on QIP schemes. As examples, we consider the distributed QIP protocol
proposed by Barrett and Kok, and Hong-Ou-Mandel interference which lies at the
heart of many linear optical schemes for quantum computing. In the distributed
QIP protocol, we find that the fidelity of entangled qubit states depends
crucially on the time resolution of single photon detectors. In particular,
there is no reduction in the fidelity when an ideal detector model is assumed,
while reduced fidelities may be encountered when using realistic detectors with
a finite response time. We obtain similar results in the case of Hong-Ou-Mandel
interference -- with perfect detectors, a modified version of quantum
interference is seen, and the visibility of the interference pattern is reduced
as the detector time resolution is reduced. Our findings indicate that problems
due to frequency mismatch can be overcome, provided sufficiently fast detectors
are available.Comment: 14 pages, 8 figures. Comments welcome. v2: Minor changes. v3: Cleaned
up 3 formatting error
Tomography of an optomechanical oscillator via parametrically amplified position measurement
We propose a protocol for quantum state tomography of nonclassical states in
optomechanical systems. Using a parametric drive, the procedure overcomes the
challenges of weak optomechanical coupling, poor detection efficiency, and
thermal noise to enable high efficiency homodyne measurement. Our analysis is
based on the analytic description of the generalized measurement that is
performed when optomechanical position measurement competes with thermal noise
and a parametric drive. The proposed experimental procedure is numerically
simulated in realistic parameter regimes, which allows us to show that
tomographic reconstruction of otherwise unverifiable nonclassical states is
made possible.Comment: 37 pages, 5 figures, comments welcome. Published versio
Stochastic simulations of conditional states of partially observed systems, quantum and classical
In a partially observed quantum or classical system the information that we
cannot access results in our description of the system becoming mixed even if
we have perfect initial knowledge. That is, if the system is quantum the
conditional state will be given by a state matrix and if classical
the conditional state will be given by a probability distribution
where is the result of the measurement. Thus to determine the evolution of
this conditional state under continuous-in-time monitoring requires an
expensive numerical calculation. In this paper we demonstrating a numerical
technique based on linear measurement theory that allows us to determine the
conditional state using only pure states. That is, our technique reduces the
problem size by a factor of , the number of basis states for the system.
Furthermore we show that our method can be applied to joint classical and
quantum systems as arises in modeling realistic measurement.Comment: 16 pages, 11 figure
Engineering Quantum Jump Superoperators for Single Photon Detectors
We study the back-action of a single photon detector on the electromagnetic
field upon a photodetection by considering a microscopic model in which the
detector is constituted of a sensor and an amplification mechanism. Using the
quantum trajectories approach we determine the Quantum Jump Superoperator (QJS)
that describes the action of the detector on the field state immediately after
the photocount. The resulting QJS consists of two parts: the bright counts
term, representing the real photoabsorptions, and the dark counts term,
representing the amplification of intrinsic excitations inside the detector.
First we compare our results for the counting rates to experimental data,
showing a good agreement. Then we point out that by modifying the field
frequency one can engineer the form of QJS, obtaining the QJS's proposed
previously in an ad hoc manner
The effect of realistic equations of state and general relativity on the "snowplow" model for pulsar glitches
Many pulsars are observed to "glitch", i.e. show sudden jumps in their
rotational frequency , some of which can be as large as in a subset of pulsars known as giant
glitchers. Recently Pizzochero (2011) has shown that an analytic model based on
realistic values for the pinning forces in the crust and for the angular
momentum transfer in the star can describe the average properties of giant
glitches, such as the inter-glitch waiting time, the step in frequency and that
in frequency derivative. In this paper we extend the model (originally
developed in Newtonian gravity and for a polytropic equation of state) to
realistic backgrounds obtained by integrating the relativistic equations of
stellar structure and using physically motivated equations of state to describe
matter in the neutron star. We find that this more detailed treatment still
reproduces the main features of giant glitches in the Vela pulsar and allows us
to set constraints on the equation of state. In particular we find that stiffer
equations of state are favoured and that it is unlikely that the Vela pulsar
has a high mass (larger than ).Comment: 15 pages, 8 figures, submitted to MNRA
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