3,363 research outputs found
Effect of atomic beam alignment on photon correlation measurements in cavity QED
Quantum trajectory simulations of a cavity QED system comprising an atomic
beam traversing a standing-wave cavity are carried out. The delayed photon
coincident rate for forwards scattering is computed and compared with the
measurements of Rempe et al. [Phys. Rev. Lett. 67, 1727 (1991)] and Foster et
al. [Phys. Rev. A 61, 053821 (2000)]. It is shown that a moderate atomic beam
misalignment can account for the degradation of the predicted correlation. Fits
to the experimental data are made in the weak-field limit with a single
adjustable parameter--the atomic beam tilt from perpendicular to the cavity
axis. Departures of the measurement conditions from the weak-field limit are
discussed.Comment: 15 pages and 13 figure
Generating single-mode behavior in fiber-coupled optical cavities
We propose to turn two resonant distant cavities effectively into one by
coupling them via an optical fiber which is coated with two-level atoms
[Franson et al., Phys. Rev. A 70, 062302 (2004)]. The purpose of the atoms is
to destructively measure the evanescent electric field of the fiber on a time
scale which is long compared to the time it takes a photon to travel from one
cavity to the other. Moreover, the boundary conditions imposed by the setup
should support a small range of standing waves inside the fiber, including one
at the frequency of the cavities. In this way, the fiber provides an additional
decay channel for one common cavity field mode but not for the other. If the
corresponding decay rate is sufficiently large, this mode decouples effectively
from the system dynamics. A single non-local resonator mode is created.Comment: 13 pages, 6 figures, final version, accepted for publicatio
Unconditional Pointer States from Conditional Master Equations
When part of the environment responsible for decoherence is used to extract
information about the decohering system, the preferred {\it pointer states}
remain unchanged. This conclusion -- reached for a specific class of models --
is investigated in a general setting of conditional master equations using
suitable generalizations of predictability sieve. We also find indications that
the einselected states are easiest to infer from the measurements carried out
on the environment.Comment: 4 pages, 3 .eps figures; final version to appear in Phys.Rev.Let
Stationary inversion of a two level system coupled to an off-resonant cavity with strong dissipation
We present an off-resonant excitation scheme that realizes pronounced
stationary inversion in a two level system. The created inversion exploits a
cavity-assisted two photon resonance to enhance the multi-photon regime of
nonlinear cavity QED and survives even in a semiconductor environment, where
the cavity decay rate is comparable to the cavity-dot coupling rate. Exciton
populations of greater than 0.75 are obtained in the presence of realistic
decay and pure dephasing. Quantum trajectory simulations and quantum master
equation calculations help elucidate the underlying physics and delineate the
limitations of a simplified rate equation model. Experimental signatures of
inversion and multi-photon cavity QED are predicted in the fluorescence
intensity and second-order correlation function measured as a function of drive
power.Comment: 4 page lette
Non-Markovian master equation for a damped oscillator with time-varying parameters
We derive an exact non-Markovian master equation that generalizes the
previous work [Hu, Paz and Zhang, Phys. Rev. D {\bf 45}, 2843 (1992)] to damped
harmonic oscillators with time-varying parameters. This is achieved by
exploiting the linearity of the system and operator solution in Heisenberg
picture. Our equation governs the non-Markovian quantum dynamics when the
system is modulated by external devices. As an application, we apply our
equation to parity kick decoupling problems. The time-dependent dissipative
coefficients in the master equation are shown to be modified drastically when
the system is driven by pulses. For coherence protection to be effective,
our numerical results indicate that kicking period should be shorter than
memory time of the bath. The effects of using soft pulses in an ohmic bath are
also discussed
Quantum feedback cooling of a single trapped ion in front of a mirror
We develop a theory of quantum feedback cooling of a single ion trapped in
front of a mirror. By monitoring the motional sidebands of the light emitted
into the mirror mode we infer the position of the ion, and act back with an
appropriate force to cool the ion. We derive a feedback master equation along
the lines of the quantum feedback theory developed by Wiseman and Milburn,
which provides us with cooling times and final temperatures as a function of
feedback gain and various system parameters.Comment: 15 pages, 11 Figure
Entanglement signature in the mode structure of a single photon
It is shown that entanglement, which is a quantum correlation property of at
least two subsystems, is imprinted in the mode structure of a single photon.
The photon, which is emitted by two coupled cavities, carries the information
on the concurrence of the two intracavity fields. This can be useful for
recording the entanglement dynamics of two cavity fields and for entanglement
transfer.Comment: 4 pages, 3 figure
Raman-assisted Rabi resonances in two-mode cavity QED
The dynamics of a vibronic system in a lossy two-mode cavity is studied, with
the first mode being resonant to the electronic transition and the second one
being nearly resonant due to Raman transitions. We derive analytical solutions
for the dynamics of this system. For a properly chosen detuning of the second
mode from the exact Raman resonance, we obtain conditions that are closely
related to the phenomenon of Rabi resonance as it is well known in laser
physics. Such resonances can be observed in the spontaneous emission spectra,
where the spectrum of the second mode in the case of weak Raman coupling is
enhanced substantially.Comment: 6 pages, 5 figure
Quantum State Diffusion and Time Correlation Functions
In computing the spectra of quantum mechanical systems one encounters the
Fourier transforms of time correlation functions, as given by the quantum
regression theorem for systems described by master equations. Quantum state
diffusion (QSD) gives a useful method of solving these problems by unraveling
the master equation into stochastic trajectories; but there is no generally
accepted definition of a time correlation function for a single QSD trajectory.
In this paper we show how QSD can be used to calculate these spectra directly;
by formally solving the equations which arise, we arrive at a natural
definition for a two-time correlation function in QSD, which depends explicitly
on both the stochastic noise of the particular trajectory and the time of
measurement, and which agrees in the mean with the ensemble average definition
of correlation functions.Comment: 16 pages standard LaTeX + 1 figure (uuencoded postscript) Numerous
minor revisions and clarifications. To appear in J. Mod. Optic
Non-classical photon pair generation in atomic vapours
A scheme for the generation of non-classical pairs of photons in atomic
vapours is proposed. The scheme exploits the fact that the cross correlation of
the emission of photons from the extreme transitions of a four-level cascade
system shows anti-bunching which has not been reported earlier and which is
unlike the case of the three level cascade emission which shows bunching. The
Cauchy-Schwarz inequality which is the ratio of cross-correlation to the auto
correlation function in this case is estimated to be for
controllable time delay, and is one to four orders of magnitude larger compared
to previous experiments. The choice of Doppler free geometry in addition to the
fact that at three photon resonance the excitation/deexcitation processes occur
in a very narrow frequency band, ensures cleaner signals.Comment: 18 pages, 7 figure
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