98 research outputs found
Effect of dissipation on quantum coherence
The effect of dissipation on a macroscopic superposition of quantum states is studied with use of a Markovian master-equation approach. It is shown that a superposition of two states is reduced to a mixture at a rate proportional to the separation between the two states. This underlines the difficulty of observing a superposition of macroscopic quantum states in practice
Quantum information processing via a lossy bus
We describe a method to perform two qubit measurements and logic operations
on pairs of qubits which each interact with a harmonic oscillator degree of
freedom (the \emph{bus}), but do not directly interact with one another. Our
scheme uses only weak interactions between the qubit and the bus, homodyne
measurements, and single qubit operations. In contrast to earlier schemes, the
technique presented here is extremely robust to photon loss in the bus mode,
and can function with high fidelity even when the rate of photon loss is
comparable to the strength of the qubit-bus coupling.Comment: Added more discussion on effects of noise. Typos correcte
Coherent state LOQC gates using simplified diagonal superposition resource states
In this paper we explore the possibility of fundamental tests for coherent
state optical quantum computing gates [T. C. Ralph, et. al, Phys. Rev. A
\textbf{68}, 042319 (2003)] using sophisticated but not unrealistic quantum
states. The major resource required in these gates are state diagonal to the
basis states. We use the recent observation that a squeezed single photon state
() approximates well an odd superposition of coherent
states () to address the diagonal resource
problem. The approximation only holds for relatively small and hence
these gates cannot be used in a scaleable scheme. We explore the effects on
fidelities and probabilities in teleportation and a rotated Hadamard gate.Comment: 21 pages, 12 figure
Noise in an SSET-resonator driven by an external field
We investigate the noise properties of a superconducting single electron
transistor (SSET) coupled to an harmonically driven resonator. Using a Langevin
equation approach, we calculate the frequency spectrum of the SSET charge and
calculate its effect on the resonator field. We find that the heights of the
peaks in the frequency spectra depend sensitively on the amplitude of the
resonator oscillation and hence suggest that the heights of these peaks could
act as a sensitive signal for detecting the small changes in the amplitude of
the drive. The previously known results for the effective amplitude-dependent
damping and temperature provided by the SSET for the case of a low frequency
resonator are generalized for all resonator frequencies.Comment: 11 pages. Accepted for PR
Quantum entanglement between a nonlinear nanomechanical resonator and a microwave field
We consider a theoretical model for a nonlinear nanomechanical resonator
coupled to a superconducting microwave resonator. The nanomechanical resonator
is driven parametrically at twice its resonance frequency, while the
superconducting microwave resonator is driven with two tones that differ in
frequency by an amount equal to the parametric driving frequency. We show that
the semi-classical approximation of this system has an interesting fixed point
bifurcation structure. In the semi-classical dynamics a transition from stable
fixed points to limit cycles is observed as one moves from positive to negative
detuning. We show that signatures of this bifurcation structure are also
present in the full dissipative quantum system and further show that it leads
to mixed state entanglement between the nanomechanical resonator and the
microwave cavity in the dissipative quantum system that is a maximum close to
the semi-classical bifurcation. Quantum signatures of the semi-classical
limit-cycles are presented.Comment: 36 pages, 18 figure
Improved fidelity of triggered entangled photons from single quantum dots
We demonstrate the on-demand emission of polarisation-entangled photon pairs
from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs
planar microcavity. Improvements in the sample design blue shifts the wetting
layer to reduce the contribution of background light in the measurements.
Results presented show that >70% of the detected photon pairs are entangled.
The high fidelity of the (|HxxHx>+|VxxVx>)/2^0.5 state that we determine is
sufficient to satisfy numerous tests for entanglement. The improved quality of
entanglement represents a significant step towards the realisation of a
practical quantum dot source compatible with applications in quantum
information.Comment: 9 pages. Paper is available free of charge at
http://www.iop.org/EJ/abstract/1367-2630/8/2/029/, see also 'A semiconductor
source of triggered entangled photon pairs', R. M. Stevenson et al., Nature
439, 179 (2006
Analysis of a Quantum Measurement
A specific measurement model based on quantum nondemolition monitoring of oscillator quanta is used to illustrate the essential elements of the quantum theory of measurement, viz., state reduction and existence of a ‘‘pointer basis,’’ recently discussed by Zurek. The form of the interaction between the meter and its environment determines the system observable recorded by the meter. In our model this quantity is the square of the oscillator’s quanta. The corresponding pointer basis states are the meter coherent states. The more accurate the measurement the more excited (and classical) these states become. We show explicitly that the state reduction of the system during measurement is due to nonunitary evolution resulting from the meter-environment coupling. A particular realization of the environment as a photoelectron counter is discussed
Synchronization of many nano-mechanical resonators coupled via a common cavity field
Using amplitude equations, we show that groups of identical nano-mechanical
resonators, interacting with a common mode of a cavity microwave field,
synchronize to form a single mechanical mode which couples to the cavity with a
strength dependent on the square sum of the individual mechanical-microwave
couplings. Classically this system is dominated by periodic behaviour which,
when analyzed using amplitude equations, can be shown to exhibit
multi-stability. In contrast groups of sufficiently dissimilar nano-mechanical
oscillators may lose synchronization and oscillate out of phase at
significantly higher amplitudes. Further the method by which synchronization is
lost resembles that for large amplitude forcing which is not of the Kuramoto
form.Comment: 23 pages, 11 figure
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
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