3,628 research outputs found
Optimization of the transmission of observable expectation values and observable statistics in Continuous Variable Teleportation
We analyze the statistics of observables in continuous variable quantum
teleportation in the formalism of the characteristic function. We derive
expressions for average values of output state observables in particular
cumulants which are additive in terms of the input state and the resource of
teleportation. Working with Squeezed Bell-like states, which may be optimized
in a free parameter for better teleportation performance we discuss the
relation between resources optimal for fidelity and for different observable
averages. We obtain the values of the free parameter which optimize the central
momenta and cumulants up to fourth order. For the cumulants the distortion
between in and out states due to teleportation depends only on the resource. We
obtain optimal parameters for the second and fourth order cumulants which do
not depend on the squeezing of the resource. The second order central momenta
which is equal to the second order cumulants and the photon number average are
optimized by the same resource. We show that the optimal fidelity resource,
found in reference (Phys. Rev. A {\bf 76}, 022301 (2007)) to depend also on the
characteristics of input, tends for high squeezing to the resource which
optimizes the second order momenta. A similar behavior is obtained for the
resource which optimizes the photon statistics which is treated here using the
sum of the squared differences in photon probabilities of input and output
states as the distortion measure. This is interpreted to mean that the
distortions associated to second order momenta dominates the behavior of the
output state for large squeezing of the resource. Optimal fidelity and optimal
photon statistics resources are compared and is shown that for mixtures of Fock
states they are equivalent.Comment: 25 pages, 11 figure
Electrostatic control of quantum dot entanglement induced by coupling to external reservoirs
We propose a quantum transport experiment to prepare and measure
charge-entanglement between two electrostatically defined quantum dots.
Coherent population trapping, as realized in cavity quantum electrodynamics,
can be carried out by using a third quantum dot to play the role of the optical
cavity. In our proposal, a pumping which is quantum mechanically
indistinguishable for the quantum dots drives the system into a state with a
high degree of entanglement. The whole effect can be switched on and off by
means of a gate potential allowing both state preparation and entanglement
detection by simply measuring the total current.Comment: 5 pages, 4 figures, Latex2e with EPL macros, to appear in Europhysics
Letter
Diffraction limit of the sub-Planck structures
The orthogonality of cat and displaced cat states, underlying Heisenberg
limited measurement in quantum metrology, is studied in the limit of large
number of states. The asymptotic expression for the corresponding state overlap
function, controlled by the sub-Planck structures arising from phase space
interference, is obtained exactly. The validity of large phase space support,
in which context the asymptotic limit is achieved, is discussed in detail. For
large number of coherent states, uniformly located on a circle, it identically
matches with the diffraction pattern for a circular ring with uniform angular
source strength. This is in accordance with the van Cittert-Zernike theorem,
where the overlap function, similar to the mutual coherence function matches
with a diffraction pattern.Comment: 5 pages, 3 figure
Quantum bistability and spin current shot noise of a single quantum dot coupled to an optical microcavity
Here we explore spin dependent quantum transport through a single quantum dot
coupled to an optical microcavity. The spin current is generated by electron
tunneling between a single doped reservoir and the dot combined with intradot
spin flip transitions induced by a quantized cavity mode. In the limit of
strong Coulomb blockade, this model is analogous to the Jaynes-Cummings model
in quantum optics and generates a pure spin current in the absence of any
charge current. Earlier research has shown that in the classical limit where a
large number of such dots interact with the cavity field, the spin current
exhibits bistability as a function of the laser amplitude that drives the
cavity. We show that in the limit of a single quantum dot this bistability
continues to be present in the intracavity photon statistics. Signatures of the
bistable photon statistics manifest themselves in the frequency dependent shot
noise of the spin current despite the fact that the quantum mechanical average
spin current no longer exhibits bistability. Besides having significance for
future quantum dot based optoelectronic devices, our results shed light on the
relation between bistability, which is traditionally viewed as a classical
effect, and quantum mechanics
Entanglement of distant optomechanical systems
We theoretically investigate the possibility to generate non-classical states
of optical and mechanical modes of optical cavities, distant from each other. A
setup comprised of two identical cavities, each with one fixed and one movable
mirror and coupled by an optical fiber, is studied in detail. We show that with
such a setup there is potential to generate entanglement between the distant
cavities, involving both optical and mechanical modes. The scheme is robust
with respect to dissipation, and nonlocal correlations are found to exist in
the steady state at finite temperatures.Comment: 12 pages (published with minor modifications
Boson-Fermion coherence in a spherically symmetric harmonic trap
We consider the photoassociation of a low-density gas of quantum-degenerate
trapped fermionic atoms into bosonic molecules in a spherically symmetric
harmonic potential. For a dilute system and the photoassociation coupling
energy small compared to the level separation of the trap, only those fermions
in the single shell with Fermi energy are coupled to the bosonic molecular
field. Introducing a collective pseudo-spin operator formalism we show that
this system can then be mapped onto the Tavis-Cummings Hamiltonian of quantum
optics, with an additional pairing interaction. By exact diagonalization of the
Hamiltonian, we examine the ground state and low excitations of the Bose-Fermi
system, and study the dynamics of the coherent coupling between atoms and
molecules. In a semiclassical description of the system, the pairing
interaction between fermions is shown to result in a self-trapping transition
in the photoassociation, with a sudden suppression of the coherent oscillations
between atoms and molecules. We also show that the full quantum dynamics of the
system is dominated by quantum fluctuations in the vicinity of the
self-trapping solution.Comment: 16 pages, 14 figure
A condition for any realistic theory of quantum systems
In quantum physics, the density operator completely describes the state.
Instead, in classical physics the mean value of every physical quantity is
evaluated by means of a probability distribution. We study the possibility to
describe pure quantum states and events with classical probability
distributions and conditional probabilities and prove that the distributions
can not be quadratic functions of the quantum state. Some examples are
considered. Finally, we deal with the exponential complexity problem of quantum
physics and introduce the concept of classical dimension for a quantum system
Theory versus experiment for vacuum Rabi oscillations in lossy cavities
The 1996 Brune {\it et al.} experiment on vacuum Rabi oscillation is analyzed
by means of alternative models of atom-reservoir interaction. Agreement with
experimental Rabi oscillation data can be obtained if one defines jump
operators in the dressed-state basis, and takes into account thermal
fluctuations between dressed states belonging to the same manifold. Such
low-frequency transitions could be ignored in a closed cavity, but the cavity
employed in the experiment was open, which justifies our assumption. The cavity
quality factor corresponding to the data is , whereas
reported in the experiment was . The rate of decoherence arising
from opening of the cavity can be of the same order as an analogous correction
coming from finite time resolution (formally equivalent to
collisional decoherence). Peres-Horodecki separability criterion shows that the
rate at which the atom-field state approaches a separable state is controlled
by fluctuations between dressed states from the same manifold, and not by the
rate of transitions towards the ground state. In consequence, improving the
factor we do not improve the coherence properties of the cavity.Comment: typo in eq. (60) corrected; (older comments: 14 figures (1 added),
value of Q improved, a section on the Peres-Horodecki test of separability
added, various small improvements; v3 includes discussion of finite time
resolution, v4 includes microscopic derivation of the master equation
Thermodynamics of quantum jump trajectories in systems driven by classical fluctuations
The large-deviation method can be used to study the measurement trajectories
of open quantum systems. For optical arrangements this formalism allows to
describe the long time properties of the (non-equilibrium) photon counting
statistics in the context of a (equilibrium) thermodynamic approach defined in
terms of dynamical phases and transitions between them in the trajectory space
[J.P. Garrahan and I. Lesanovsky, Phys. Rev. Lett. 104, 160601 (2010)]. In this
paper, we study the thermodynamic approach for fluorescent systems coupled to
complex reservoirs that induce stochastic fluctuations in their dynamical
parameters. In a fast modulation limit the thermodynamics corresponds to that
of a Markovian two-level system. In a slow modulation limit, the thermodynamic
properties are equivalent to those of a finite system that in an infinite-size
limit is characterized by a first-order transition. The dynamical phases
correspond to different intensity regimes, while the size of the system is
measured by the transition rate of the bath fluctuations. As a function of a
dimensionless intensive variable, the first and second derivative of the
thermodynamic potential develop an abrupt change and a narrow peak
respectively. Their scaling properties are consistent with a double-Gaussian
probability distribution of the associated extensive variable.Comment: 12 pages, 3 figure
Dynamical multistability in high-finesse micromechanical optical cavities
We analyze the nonlinear dynamics of a high-finesse optical cavity in which
one mirror is mounted on a flexible mechanical element. We find that this
system is governed by an array of dynamical attractors, which arise from
phase-locking between the mechanical oscillations of the mirror and the ringing
of the light intensity in the cavity. We describe an analytical approximation
to map out the diagram of attractors in parameter space, derive the slow
amplitude dynamics of the system, including thermally activated hopping between
different attractors, and suggest a scheme for exploiting the dynamical
multistability in the measurement of small displacements.Comment: 5 pages, 4 figure
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