3,585 research outputs found
Non-classical Photon Statistics For Two-mode Optical Fields
The non-classical property of subpoissonian photon statistics is extended
from one to two-mode electromagnetic fields, incorporating the physically
motivated property of invariance under passive unitary transformations.
Applications to squeezed coherent states, squeezed thermal states, and
superposition of coherent states are given. Dependences of extent of
non-classical behaviour on the independent squeezing parameters are graphically
displayed.Comment: 15 pages, RevTex, 5 figures, available by sending email to
[email protected]
Open timelike curves violate Heisenberg's uncertainty principle
Toy models for quantum evolution in the presence of closed timelike curves
(CTCs) have gained attention in the recent literature due to the strange
effects they predict. The circuits that give rise to these effects appear quite
abstract and contrived, as they require non-trivial interactions between the
future and past which lead to infinitely recursive equations. We consider the
special case in which there is no interaction inside the CTC, referred to as an
open timelike curve (OTC), for which the only local effect is to increase the
time elapsed by a clock carried by the system. Remarkably, circuits with access
to OTCs are shown to violate Heisenberg's uncertainty principle, allowing
perfect state discrimination and perfect cloning of coherent states. The model
is extended to wave-packets and smoothly recovers standard quantum mechanics in
an appropriate physical limit. The analogy with general relativistic
time-dilation suggests that OTCs provide a novel alternative to existing
proposals for the behaviour of quantum systems under gravity
Low-energy electron scattering by tetrahydrofuran
Cross sections for elastic scattering of low-energy electrons by tetrahydrofuran, a prototype for the furanose ring found in the backbone of DNA, have been measured and calculated over a wide energy range, with an emphasis on energies below 6 eV, where previous data are scarce. The measurements employ a thin-aperture version of the relative-flow method, while the calculations employ the Schwinger multichannel method with an extensive treatment of polarization effects. Comparisons with earlier results, both experimental and theoretical, are presented and discussed. A proper accounting for the strong permanent electric dipole of tetrahydrofuran is found to be essential to obtaining reliable cross sections, especially at energies below 5 eV
Paired atom laser beams created via four-wave mixing
A method to create paired atom laser beams from a metastable helium atom
laser via four-wave mixing is demonstrated. Radio frequency outcoupling is used
to extract atoms from a Bose Einstein condensate near the center of the
condensate and initiate scattering between trapped and untrapped atoms. The
unequal strengths of the interactions for different internal states allows an
energy-momentum resonance which leads to the creation of pairs of atoms
scattered from the zero-velocity condensate. The resulting scattered beams are
well separated from the main atom laser in the 2-dimensional transverse atom
laser profile. Numerical simulations of the system are in good agreement with
the observed atom laser spatial profiles, and indicate that the scattered beams
are generated by a four-wave mixing process, suggesting that the beams are
correlated.Comment: 5 pages, 3 figure
The Quantum State of an Ideal Propagating Laser Field
We give a quantum information-theoretic description of an ideal propagating
CW laser field and reinterpret typical quantum-optical experiments in light of
this. In particular we show that contrary to recent claims [T. Rudolph and B.
C. Sanders, Phys. Rev. Lett. 87, 077903 (2001)], a conventional laser can be
used for quantum teleportation with continuous variables and for generating
continuous-variable entanglement. Optical coherence is not required, but phase
coherence is. We also show that coherent states play a priveleged role in the
description of laser light.Comment: 4 pages RevTeX, to appear in PRL. For an extended version see
quant-ph/011115
Microscopic theory of phonon-induced effects on semiconductor quantum dot decay dynamics in cavity QED
We investigate the influence of the electron-phonon interaction on the decay
dynamics of a quantum dot coupled to an optical microcavity. We show that the
electron-phonon interaction has important consequences on the dynamics,
especially when the quantum dot and cavity are tuned out of resonance, in which
case the phonons may add or remove energy leading to an effective non-resonant
coupling between quantum dot and cavity. The system is investigated using two
different theoretical approaches: (i) a second-order expansion in the bare
phonon coupling constant, and (ii) an expansion in a polaron-photon coupling
constant, arising from the polaron transformation which allows an accurate
description at high temperatures. In the low temperature regime we find
excellent agreement between the two approaches. An extensive study of the
quantum dot decay dynamics is performed, where important parameter dependencies
are covered. We find that in general the electron-phonon interaction gives rise
to a greatly increased bandwidth of the coupling between quantum dot and
cavity. At low temperature an asymmetry in the quantum dot decay rate is
observed, leading to a faster decay when the quantum dot has a larger energy
than to the cavity. We explain this as due to the absence of phonon absorption
processes. Furthermore, we derive approximate analytical expressions for the
quantum dot decay rate, applicable when the cavity can be adiabatically
eliminated. The expressions lead to a clear interpretation of the physics and
emphasizes the important role played by the effective phonon density,
describing the availability of phonons for scattering, in quantum dot decay
dynamics. Based on the analytical expressions we present the parameter regimes
where phonon effects are expected to be important. Also, we include all
technical developments in appendices.Comment: published PRB version, comments are very welcom
Generation of directional, coherent matter beams through dynamical instabilities in Bose-Einstein condensates
We present a theoretical analysis of a coupled, two-state Bose-Einstein
condensate with non-equal scattering lengths, and show that dynamical
instabilities can be excited. We demonstrate that these instabilities are
exponentially amplified resulting in highly-directional,
oppositely-propagating, coherent matter beams at specific momenta. To
accomplish this we prove that the mean field of our system is periodic, and
extend the standard Bogoliubov approach to consider a time-dependent, but
cyclic, background. This allows us to use Floquet's theorem to gain analytic
insight into such systems, rather than employing the usual Bogoliubov-de Gennes
approach, which is usually limited to numerical solutions. We apply our theory
to the metastable Helium atom laser experiment of Dall et al. [Phys. Rev. A 79,
011601(R) (2009)] and show it explains the anomalous beam profiles they
observed. Finally we demonstrate the paired particle beams will be
EPR-entangled on formation.Comment: Corrected reference
The Mach-Zehnder and the Teleporter
We suggest a self-testing teleportation configuration for photon q-bits based
on a Mach-Zehnder interferometer. That is, Bob can tell how well the input
state has been teleported without knowing what that input state was. One could
imagine building a "locked" teleporter based on this configuration. The
analysis is performed for continuous variable teleportation but the arrangement
could equally be applied to discrete manipulations.Comment: 4 pages, 5 figure
Self-induced spatial dynamics to enhance spin squeezing via one-axis twisting in a two component Bose-Einstein condensate
We theoretically investigate a scheme to enhance relative number squeezing and spin squeezing in a two- component Bose-Einstein condensate (BEC) by utilizing the inherent mean-field dynamics of the condensate. Due to the asymmetry in the scattering lengths, the two components exhibit large density oscillations where they spatially separate and recombine. The effective nonlinearity responsible for the squeezing is increased by up to 3 orders of magnitude when the two components spatially separate. We perform a multimode simulation of the system using the truncated Wigner method and show that this method can be used to create significant squeezing in systems where the effective nonlinearity would ordinarily be too small to produce any significant squeezing in sensible time frames, and we show that strong spatial dynamics resulting from large particle numbers aren’t necessarily detrimental to generating squeezing. We develop a simplified semianalytic model that gives good agreement with our multimode simulation and will be useful for predicting squeezing in a range of different systems
Entanglement of Dirac fields in non-inertial frames
We analyze the entanglement between two modes of a free Dirac field as seen
by two relatively accelerated parties. The entanglement is degraded by the
Unruh effect and asymptotically reaches a non-vanishing minimum value in the
infinite acceleration limit. This means that the state always remains entangled
to a degree and can be used in quantum information tasks, such as
teleportation, between parties in relative uniform acceleration. We analyze our
results from the point of view afforded by the phenomenon of entanglement
sharing and in terms of recent results in the area of multi-qubit
complementarity.Comment: 15 pages, with 8 figures (Mar 2006); accepted to Physical Review A,
July 2006 - slightly revise
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