101 research outputs found
Towards the Heisenberg limit in microwave photon detection by a qubit array
Using an analytically solvable model, we show that a qubit array-based
detector allows to achieve the fundamental Heisenberg limit in detecting single
photons. In case of superconducting qubits, this opens new opportunities for
quantum sensing and communications in the important microwave range.Comment: 6 pages, 3 figure
Ghost imaging using homodyne detection
We present a theoretical study of ghost imaging based on correlated beams
arising from parametric down-conversion, and which uses balanced homodyne
detection to measure both the signal and idler fields. We analytically show
that the signal-idler correlations contain the full amplitude and phase
information about an object located in the signal path, both in the near-field
and the far-field case. To this end we discuss how to optimize the optical
setups in the two imaging paths, including the crucial point regarding how to
engineer the phase of the idler local oscillator as to observe the desired
orthogonal quadrature components of the image. We point out an inherent link
between the far-field bandwidth and the near-field resolution of the reproduced
image, determined by the bandwidth of the source of the correlated beams.
However, we show how to circumvent this limitation by using a spatial averaging
technique which dramatically improves the imaging bandwidth of the far-field
correlations as well as speeds up the convergence rate. The results are backed
up by numerical simulations taking into account the finite size and duration of
the pump pulse.Comment: 17 pages, 10 figures, submitted to Phys. Rev.
Kinetic theory and dynamic structure factor of a condensate in the random phase approximation
We present the microscopic kinetic theory of a homogeneous dilute Bose
condensed gas in the generalized random phase approximation (GRPA), which
satisfies the following requirements: 1) the mass, momentum and energy
conservation laws; 2) the H-theorem; 3) the superfluidity property and 4) the
recovery of the Bogoliubov theory at zero temperature \cite{condenson}. In this
approach, the condensate influences the binary collisional process between the
two normal atoms, in the sense that their interaction force results from the
mediation of a Bogoliubov collective excitation traveling throughout the
condensate. Furthermore, as long as the Bose gas is stable, no collision
happens between condensed and normal atoms. In this paper, we show how the
kinetic theory in the GRPA allows to calculate the dynamic structure factor at
finite temperature and when the normal and superfluid are in a relative motion.
The obtained spectrum for this factor provides a prediction which, compared to
the experimental results, allows to validate the GRPA.
PACS numbers:03.75.Hh, 03.75.Kk, 05.30.-dComment: 6 pages, 1 figures, QFS2004 conferenc
The optimal cloning of quantum coherent states is non-Gaussian
We consider the optimal cloning of quantum coherent states with single-clone
and joint fidelity as figures of merit. Both optimal fidelities are attained
for phase space translation covariant cloners. Remarkably, the joint fidelity
is maximized by a Gaussian cloner, whereas the single-clone fidelity can be
enhanced by non-Gaussian operations: a symmetric non-Gaussian 1-to-2 cloner can
achieve a single-clone fidelity of approximately 0.6826, perceivably higher
than the optimal fidelity of 2/3 in a Gaussian setting. This optimal cloner can
be realized by means of an optical parametric amplifier supplemented with a
particular source of non-Gaussian bimodal states. Finally, we show that the
single-clone fidelity of the optimal 1-to-infinity cloner, corresponding to a
measure-and-prepare scheme, cannot exceed 1/2. This value is achieved by a
Gaussian scheme and cannot be surpassed even with supplemental bound entangled
states.Comment: 4 pages, 2 figures, revtex; changed title, extended list of authors,
included optical implementation of optimal clone
Stochastic effects at ripple formation processes in anisotropic systems with multiplicative noise
We study pattern formation processes in anisotropic system governed by the
Kuramoto-Sivashinsky equation with multiplicative noise as a generalization of
the Bradley-Harper model for ripple formation induced by ion bombardment. For
both linear and nonlinear systems we study noise induced effects at ripple
formation and discuss scaling behavior of the surface growth and roughness
characteristics. It was found that the secondary parameters of the ion beam
(beam profile and variations of an incidence angle) can crucially change the
topology of patterns and the corresponding dynamics
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