669 research outputs found
Conditional teleportation using optical squeezers and photon counting
We suggest a scheme of using two-mode squeezed vacuum for conditional
teleportation of quantum states of optical field. Alice mixes the input state
with one of the squeezed modes on another squeezing device and detects the
output photon numbers. The result is then communicated to Bob who shifts the
photon number of his part accordingly. This is a principally realizable
modification of the recent scheme [G.J. Milburn and S.L. Braunstein, Phys. Rev.
A 60, 937 (1999)] where measurements of photon number difference and phase sum
are considered. We show that for some classes of states this method can yield
very high fidelity of teleportation, nevertheless, the success probability may
be limited.Comment: 5 pages, 4 figures; notations simplified, more explicit explanatio
Entangled light in transition through the generation threshold
We investigate continuous variable entangling resources on the base of
two-mode squeezing for all operational regimes of a nondegenerate optical
parametric oscillator with allowance for quantum noise of arbitrary level. The
results for the quadrature variances of a pair of generated modes are obtained
by using the exact steady-state solution of Fokker-Planck equation for the
complex P-quasiprobability distribution function. We find a simple expression
for the squeezed variances in the near-threshold range and conclude that the
maximal two-mode squeezing reaches 50% relative to the level of vacuum
fluctuations and is achieved at the pump field intensity close to the
generation threshold. The distinction between the degree of two-mode squeezing
for monostable and bistable operational regimes is cleared up.Comment: 7 pages, 4 figures; Content changed: more details added to the
discussion. To be published in Phys. Rev.
Noiseless Linear Amplification and Distillation of Entanglement
The idea of signal amplification is ubiquitous in the control of physical
systems, and the ultimate performance limit of amplifiers is set by quantum
physics. Increasing the amplitude of an unknown quantum optical field, or more
generally any harmonic oscillator state, must introduce noise. This linear
amplification noise prevents the perfect copying of the quantum state, enforces
quantum limits on communications and metrology, and is the physical mechanism
that prevents the increase of entanglement via local operations. It is known
that non-deterministic versions of ideal cloning and local entanglement
increase (distillation) are allowed, suggesting the possibility of
non-deterministic noiseless linear amplification. Here we introduce, and
experimentally demonstrate, such a noiseless linear amplifier for
continuous-variables states of the optical field, and use it to demonstrate
entanglement distillation of field-mode entanglement. This simple but powerful
circuit can form the basis of practical devices for enhancing quantum
technologies. The idea of noiseless amplification unifies approaches to cloning
and distillation, and will find applications in quantum metrology and
communications.Comment: Submitted 10 June 200
Von Bezold assimilation effect reverses in stereoscopic conditions
Lightness contrast and lightness assimilation are opposite phenomena: in contrast,
grey targets appear darker when bordering bright surfaces (inducers) rather than dark ones; in
assimilation, the opposite occurs. The question is: which visual process favours the occurrence
of one phenomenon over the other? Researchers provided three answers to this question. The
first asserts that both phenomena are caused by peripheral processes; the second attributes their
occurrence to central processes; and the third claims that contrast involves central processes,
whilst assimilation involves peripheral ones. To test these hypotheses, an experiment on an IT
system equipped with goggles for stereo vision was run. Observers were asked to evaluate the
lightness of a grey target, and two variables were systematically manipulated: (i) the apparent
distance of the inducers; and (ii) brightness of the inducers. The retinal stimulation was kept
constant throughout, so that the peripheral processes remained the same. The results show that
the lightness of the target depends on both variables. As the retinal stimulation was kept constant, we
conclude that central mechanisms are involved in both lightness contrast and lightness assimilation
On quantum teleportation with beam-splitter-generated entanglement
Following the lead of Cochrane, Milburn, and Munro [Phys. Rev. A {\bf 62},
062307 (2000)], we investigate theoretically quantum teleportation by means of
the number-sum and phase-difference variables. We study Fock-state entanglement
generated by a beam splitter and show that two-mode Fock-state inputs can be
entangled by a beam splitter into close approximations of maximally entangled
eigenstates of the phase difference and the photon-number sum
(Einstein-Podolsky-Rosen -- EPR -- states). Such states could be experimentally
feasible with on-demand single-photon sources. We show that the teleportation
fidelity can reach near unity when such ``quasi-EPR'' states are used as the
quantum channel.Comment: 7 pages (two-column), 7 figures, submitted to Phys. Rev. A. Text
unmodified, postscript error correcte
Experimental Vacuum Squeezing in Rubidium Vapor via Self-Rotation
We report the generation of optical squeezed vacuum states by means of
polarization self-rotation in rubidium vapor following a proposal by Matsko et
al. [Phys. Rev. A 66, 043815 (2002)]. The experimental setup, involving in
essence just a diode laser and a heated rubidium gas cell, is simple and easily
scalable. A squeezing of 0.85+-0.05 dB was achieved
Stable nondegenerate optical parametric oscillation at degenerate frequencies in Na:KTP
We report the realization of a light source specifically designed for the
generation of bright continuous-variable entangled beams and for
Heisenberg-limited inteferometry. The source is a nondegenerate, single-mode,
continuous-wave optical parametric oscillator in Na:KTP, operated at frequency
degeneracy and just above threshold, which is also of interest for the study of
critical fluctuations at the transition point. The residual
frequency-difference jitter is 150 kHz for a 3 MHz cold cavity half-width
at half maximum. We observe 4 dB of photon-number-difference squeezing at 200
kHz. The Na:KTP crystal is noncritically phase-matched for a 532 nm pump and
polarization crosstalk is therefore practically nonexistent
Motion-light parametric amplifier and entanglement distributor
We propose a scheme for entangling the motional mode of a trapped atom with a
propagating light field via a cavity-mediated parametric interaction. We then
show that if this light field is subsequently coupled to a second distant atom
via a cavity-mediated linear-mixing interaction, it is possible to transfer the
entanglement from the light beam to the motional mode of the second atom to
create an EPR-type entangled state of the positions and momenta of two
distantly-separated atoms.Comment: 9 pages, 8 figures, REVTe
Generation of Continuous Variable Einstein-Podolsky-Rosen Entanglement via the Kerr Nonlinearity in an Optical Fibre
We report on the generation of a continuous variable Einstein-Podolsky-Rosen
(EPR) entanglement using an optical fibre interferometer. The Kerr nonlinearity
in the fibre is exploited for the generation of two independent squeezed beams.
These interfere at a beam splitter and EPR entanglement is obtained between the
output beams. The correlation of the amplitude (phase) quadratures are measured
to be 4.0+-0.2 (4.0+-0.4) dB below the quantum noise limit. The sum criterion
for these squeezing variances 0.80+-0.03 < 2 verifies the nonseparability of
the state. The product of the inferred uncertainties for one beam 0.64+-0.08 is
well below the EPR limit of unity.Comment: RevTeX, 4 pages, 3 figures, to be published in Phys. Rev. Let
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