2,608 research outputs found
Reply on the ``Comment on `Loss-error compensation in quantum- state measurements' ''
The authors of the Comment [G. M. D'Ariano and C. Macchiavello to be
published in Phys. Rev. A, quant-ph/9701009] tried to reestablish a 0.5
efficiency bound for loss compensation in optical homodyne tomography. In our
reply we demonstrate that neither does such a rigorous bound exist nor is the
bound required for ruling out the state reconstruction of an individual system
[G. M. D'Ariano and H. P. Yuen, Phys. Rev. Lett. 76, 2832 (1996)].Comment: LaTex, 2 pages, 1 Figure; to be published in Physical Review
No quantum friction between uniformly moving plates
The Casimir forces between two plates moving parallel to each other are found
by calculating the vacuum electromagnetic stress tensor. The perpendicular
force between the plates is modified by the motion but there is no lateral
force on the plates. Electromagnetic vacuum fluctuations do not therefore give
rise to "quantum friction" in this case, contrary to previous assertions. The
result shows that the Casimir-Polder force on a particle moving at constant
speed parallel to a plate also has no lateral component.Comment: 17 pages. Final, published versio
Quantum Markov Process on a Lattice
We develop a systematic description of Weyl and Fano operators on a lattice
phase space. Introducing the so-called ghost variable even on an odd lattice,
odd and even lattices can be treated in a symmetric way. The Wigner function is
defined using these operators on the quantum phase space, which can be
interpreted as a spin phase space. If we extend the space with a dichotomic
variable, a positive distribution function can be defined on the new space. It
is shown that there exits a quantum Markov process on the extended space which
describes the time evolution of the distribution function.Comment: Lattice2003(theory
Reply to the ``Comment on `quantum backaction of optical observations on Bose-Einstein condensates' ''
In our paper we estimated the quantum backaction of dispersive imaging with
off-resonant light on Bose-Einstein condensates. We have calculated the rates
of the two processes involved, phase diffusion and depletion of the condensate.
We compare here the depletion rate obtained within our model limitations to the
Rayleigh scattering rate, both having the same physical origin: dispersive
interaction of light with matter. We show that residual absorption sets indeed
the limit of dispersive imaging.Comment: 1 page (Reply to comment
Quantum back-action of optical observations on Bose condensates
Impressive pictures of moving Bose-Einstein condensates have been taken using
phase-contrast imaging M. R. Andrews et al., Science 273, 84 (1996). We
calculate the quantum backaction of this measurement technique. We find that
phase-contrast imaging is not a quantum nondemolition measurement of the atomic
density. Instead, the condensate gets gradually depleted at a rate that is
proportional to the light intensity and to the inverse cube of the optical wave
length. The fewer atoms are condensed the higher is the required intensity to
see a picture, and, consequently, the higher is the induced backaction. To
describe the quantum physics of phase-contrast imaging we put forward a new
approach to quantum-optical propagation. We develop an effective field theory
of paraxial optics in a fully quantized atomic medium.Comment: 11 pages RevTex, 2 ps figures, revised. European Physical Journal D
(in press
Theory of elementary excitations in unstable Bose-Einstein condensates
Like classical fluids, quantum gases may suffer from hydrodynamic instabilities. Our paper develops a quantum version of the classical stability analysis in fluids, the Bogoliubov theory of elementary excitations in unstable Bose-Einstein condensates. In unstable condensates the excitation modes have complex frequencies. We derive the normalization conditions for unstable modes such that they can serve in a mode decomposition of the noncondensed component. Furthermore, we develop approximative techniques to determine the spectrum and the mode functions. Finally, we apply our theory to sonic horizons - sonic black and white holes. For sonic white holes the spectrum of unstable modes turns out to be intrinsically discrete, whereas black holes may be stable
Superantenna made of transformation media
We show how transformation media can make a superantenna that is either
completely invisible or focuses incoming light into a needle-sharp beam. Our
idea is based on representating three-dimensional space as a foliage of sheets
and performing two-dimensional conformal maps on each shee
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