1,516 research outputs found
Quantum optomechanics of a multimode system coupled via photothermal and radiation pressure force
We provide a full quantum description of the optomechanical system formed by
a Fabry-Perot cavity with a movable micro-mechanical mirror whose
center-of-mass and internal elastic modes are coupled to the driven cavity mode
by both radiation pressure and photothermal force. Adopting a quantum Langevin
description, we investigate simultaneous cooling of the micromirror elastic and
center-of-mass modes, and also the entanglement properties of the
optomechanical multipartite system in its steady state.Comment: 11 pages, 7 figure
Coherent generation of EPR-entangled light pulses mediated by a single trapped atom
We show that a single, trapped, laser-driven atom in a high-finesse optical
cavity allows for the quantum-coherent generation of entangled light pulses on
demand. Schemes for generating simultaneous and temporally separated pulse
pairs are proposed. The mechanical effect of the laser excitation on the
quantum motion of the cold trapped atom mediates the entangling interaction
between two cavity modes and between the two subsequent pulses, respectively.
The entanglement is of EPR-type, and its degree can be controlled through
external parameters. At the end of the generation process the atom is
decorrelated from the light field. Possible experimental implementations of the
proposals are discussed.Comment: 11 pages, 4 figure
Quantum-limited force measurement with an optomechanical device
We study the detection of weak coherent forces by means of an optomechanical
device formed by a highly reflecting isolated mirror shined by an intense and
highly monochromatic laser field. Radiation pressure excites a vibrational mode
of the mirror, inducing sidebands of the incident field, which are then
measured by heterodyne detection. We determine the sensitivity of such a scheme
and show that the use of an entangled input state of the two sideband modes
improves the detection, even in the presence of damping and noise acting on the
mechanical mode.Comment: 8 pages, 4 figure
Scheme for teleportation of quantum states onto a mechanical resonator
We propose an experimentally feasible scheme to teleport an unkown quantum
state onto the vibrational degree of freedom of a macroscopic mirror. The
quantum channel between the two parties is established by exploiting radiation
pressure effects.Comment: 5 pages, 2 figures, in press on PR
Quantum versus Semiclassical Description of Selftrapping: Anharmonic Effects
Selftrapping has been traditionally studied on the assumption that
quasiparticles interact with harmonic phonons and that this interaction is
linear in the displacement of the phonon. To complement recent semiclassical
studies of anharmonicity and nonlinearity in this context, we present below a
fully quantum mechanical analysis of a two-site system, where the oscillator is
described by a tunably anharmonic potential, with a square well with infinite
walls and the harmonic potential as its extreme limits, and wherein the
interaction is nonlinear in the oscillator displacement. We find that even
highly anharmonic polarons behave similar to their harmonic counterparts in
that selftrapping is preserved for long times in the limit of strong coupling,
and that the polaronic tunneling time scale depends exponentially on the
polaron binding energy. Further, in agreement, with earlier results related to
harmonic polarons, the semiclassical approximation agrees with the full quantum
result in the massive oscillator limit of small oscillator frequency and strong
quasiparticle-oscillator coupling.Comment: 10 pages, 6 figures, to appear in Phys. Rev.
Preserving entanglement under decoherence and sandwiching all separable states
Every entangled state can be perturbed, for instance by decoherence, and stay
entangled. For a large class of pure entangled states, we show how large the
perturbation can be. Our class includes all pure bipartite and all maximally
entangled states. For an entangled state, E, the constucted neighborhood of
entangled states is the region outside two parallel hyperplanes, which sandwich
the set of all separable states. The states for which these neighborhoods are
largest are the maximally entangled ones. As the number of particles, or the
dimensions of the Hilbert spaces for two of the particles increases, the
distance between two of the hyperplanes which sandwich the separable states
goes to zero. It is easy to decide if a state Q is in the neighborhood of
entangled states we construct for an entangled state E. One merely has to check
if the trace of EQ is greater than a constant which depends upon E and which we
determine.Comment: Corrected first author's e-mail address. All the rest remains
unchange
Generating continuous variable quantum codewords in the near-field atomic lithography
Recently, D. Gottesman et al. [Phys. Rev. A 64, 012310 (2001)] showed how to
encode a qubit into a continuous variable quantum system. This encoding was
realized by using non-normalizable quantum codewords, which therefore can only
be approximated in any real physical setup. Here we show how a neutral atom,
falling through an optical cavity and interacting with a single mode of the
intracavity electromagnetic field, can be used to safely encode a qubit into
its external degrees of freedom. In fact, the localization induced by a
homodyne detection of the cavity field is able to project the near-field atomic
motional state into an approximate quantum codeword. The performance of this
encoding process is then analyzed by evaluating the intrinsic errors induced in
the recovery process by the approximated form of the generated codeword.Comment: 9 pages, 5 figure
Continuous pumping and control of mesoscopic superposition state in a lossy QED cavity
Here we consider the continuous pumping of a dissipative QED cavity and
derive the time-dependent density operator of the cavity field prepared
initially as a superposition of mesoscopic coherent states. The control of the
coherence of this superposition is analyzed considering the injection of a beam
of two-level Rydberg atoms through the cavity. Our treatment is compared to
other approaches.Comment: 15 pages, 6 PostScript figures, To appear in Phys. Rev.
Assessment of a quantum phase gate operation based on nonlinear optics
We analyze in detail the proposal for a two-qubit gate for travelling
single-photon qubits recently presented by C. Ottaviani \emph{et al}. [Phys.
Rev. A \textbf{73}, 010301(R) (2006)]. The scheme is based on an ensemble of
five-level atoms coupled to two quantum and two classical light fields. The two
quantum fields undergo cross-phase modulation induced by electromagnetically
induced transparency. The performance of this two-qubit quantum phase gate for
travelling single-photon qubits is thoroughly examined in the steady-state and
transient regimes, by means of a full quantum treatment of the system dynamics.
In the steady-state regime, we find a general trade-off between the size of the
conditional phase shift and the fidelity of the gate operation. However, this
trade-off can be bypassed in the transient regime, where a satisfactory gate
operation is found to be possible, significantly reducing the gate operation
time.Comment: 12 pages, 15 figure
Quantum State Protection in Cavities
We show how an initially prepared quantum state of a radiation mode in a
cavity can be preserved for a long time using a feedback scheme based on the
injection of appropriately prepared atoms. We present a feedback scheme both
for optical cavities, which can be continuously monitored by a photodetector,
and for microwave cavities, which can be monitored only indirectly via the
detection of atoms that have interacted with the cavity field. We also discuss
the possibility of applying these methods for decoherence control in quantum
information processing.Comment: RevTex, 9 figures, submitted to Phys. Rev.
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