17,953 research outputs found
Collinear Four-Wave Mixing of Two-Component Matter Waves
We demonstrate atomic four-wave mixing of two-component matter waves in a
collinear geometry. Starting from a single-species Bose-Einstein condensate,
seed and pump modes are prepared through microwave state transfer and
state-selective Kapitza-Dirac diffraction. Four-wave mixing then populates the
initially empty output modes. Simulations based on a coupled-mode expansion of
the Gross-Pitaevskii equation are in very good agreement with the experimental
data. We show that four-wave mixing can play an important role in studies of
bosonic mixtures in optical lattices. Moreover our system should be of interest
in the context of quantum atom optics.Comment: 4 pages, 4 figures; revised version, essentially as publishe
Optically mediated nonlinear quantum optomechanics
We consider theoretically the optomechanical interaction of several
mechanical modes with a single quantized cavity field mode for linear and
quadratic coupling. We focus specifically on situations where the optical
dissipation is the dominant source of damping, in which case the optical field
can be adiabatically eliminated, resulting in effective multimode interactions
between the mechanical modes. In the case of linear coupling, the coherent
contribution to the interaction can be exploited e.g. in quantum state swapping
protocols, while the incoherent part leads to significant modifications of cold
damping or amplification from the single-mode situation. Quadratic coupling can
result in a wealth of possible effective interactions including the analogs of
second-harmonic generation and four-wave mixing in nonlinear optics, with
specific forms depending sensitively on the sign of the coupling. The
cavity-mediated mechanical interaction of two modes is investigated in two
limiting cases, the resolved sideband and the Doppler regime. As an
illustrative application of the formal analysis we discuss in some detail a
two-mode system where a Bose-Einstein condensate is optomechanically linearly
coupled to the moving end mirror of a Fabry-P\'erot cavity.Comment: 11 pages, 8 figure
Wide-bandwidth, tunable, multiple-pulse-width optical delays using slow light in cesium vapor
We demonstrate an all-optical delay line in hot cesium vapor that tunably
delays 275 ps input pulses up to 6.8 ns and 740 input ps pulses up to 59 ns
(group index of approximately 200) with little pulse distortion. The delay is
made tunable with a fast reconfiguration time (hundreds of ns) by optically
pumping out of the atomic ground states.Comment: 4 pages, 6 figure
On causality, apparent 'superluminality' and reshaping in barrier penetration
We consider tunnelling of a non-relativistic particle across a potential
barrier. It is shown that the barrier acts as an effective beam splitter which
builds up the transmitted pulse from the copies of the initial envelope shifted
in the coordinate space backwards relative to the free propagation. Although
along each pathway causality is explicitly obeyed, in special cases reshaping
can result an overall reduction of the initial envelope, accompanied by an
arbitrary coordinate shift. In the case of a high barrier the delay amplitude
distribution (DAD) mimics a Dirac -function, the transmission amplitude
is superoscillatory for finite momenta and tunnelling leads to an accurate
advancement of the (reduced) initial envelope by the barrier width. In the case
of a wide barrier, initial envelope is accurately translated into the complex
coordinate plane. The complex shift, given by the first moment of the DAD,
accounts for both the displacement of the maximum of the transmitted
probability density and the increase in its velocity. It is argued that
analysing apparent 'superluminality' in terms of spacial displacements helps
avoid contradiction associated with time parameters such as the phase time
Quantum channels in nonlinear optical processes
Quantum electrodynamics furnishes a new type of representation for the characterisation of nonlinear optical processes. The treatment elicits the detailed role and interplay of specific quantum channels, information that is not afforded by other methods. Following an illustrative application to the case of Rayleigh scattering, the method is applied to second and third harmonic generation. Derivations are given of parameters that quantify the various quantum channels and their interferences; the results are illustrated graphically. With given examples, it is shown in some systems that optical nonlinearity owes its origin to an isolated channel, or a small group of channels. © 2009 World Scientific Publishing Company
Bright squeezed vacuum in a nonlinear interferometer: frequency/temporal Schmidt-mode description
Control over the spectral properties of the bright squeezed vacuum (BSV), a
highly multimode non-classical macroscopic state of light that can be generated
through high-gain parametric down conversion, is crucial for many applications.
In particular, in several recent experiments BSV is generated in a strongly
pumped SU(1,1) interferometer to achieve phase supersensitivity, perform
broadband homodyne detection, or tailor the frequency spectrum of squeezed
light. In this work, we present an analytical approach to the theoretical
description of BSV in the frequency domain based on the Bloch-Messiah reduction
and the Schmidt-mode formalism. As a special case we consider a strongly pumped
SU(1,1) interferometer. We show that different moments of the radiation at its
output depend on the phase, dispersion and the parametric gain in a nontrivial
way, thereby providing additional insights on the capabilities of nonlinear
interferometers. In particular, a dramatic change in the spectrum occurs as the
parametric gain increases
Resolving velocity space dynamics in continuum gyrokinetics
Many plasmas of interest to the astrophysical and fusion communities are
weakly collisional. In such plasmas, small scales can develop in the
distribution of particle velocities, potentially affecting observable
quantities such as turbulent fluxes. Consequently, it is necessary to monitor
velocity space resolution in gyrokinetic simulations. In this paper, we present
a set of computationally efficient diagnostics for measuring velocity space
resolution in gyrokinetic simulations and apply them to a range of plasma
physics phenomena using the continuum gyrokinetic code GS2. For the cases
considered here, it is found that the use of a collisionality at or below
experimental values allows for the resolution of plasma dynamics with
relatively few velocity space grid points. Additionally, we describe
implementation of an adaptive collision frequency which can be used to improve
velocity space resolution in the collisionless regime, where results are
expected to be independent of collision frequency.Comment: 20 pages, 11 figures, submitted to Phys. Plasma
A symmetry analyser for non-destructive Bell state detection using EIT
We describe a method to project photonic two-qubit states onto the symmetric
and antisymmetric subspaces of their Hilbert space. This device utilizes an
ancillary coherent state, together with a weak cross-Kerr non-linearity,
generated, for example, by electromagnetically induced transparency. The
symmetry analyzer is non-destructive, and works for small values of the
cross-Kerr coupling. Furthermore, this device can be used to construct a
non-destructive Bell state detector.Comment: Final published for
Witnessed entanglement and the geometric measure of quantum discord
We establish relations between geometric quantum discord and entanglement
quantifiers obtained by means of optimal witness operators. In particular, we
prove a relation between negativity and geometric discord in the
Hilbert-Schmidt norm, which is slightly different from a previous conjectured
one [Phys. Rev. A 84, 052110 (2011)].We also show that, redefining the
geometric discord with the trace norm, better bounds can be obtained. We
illustrate our results numerically.Comment: 8 pages + 3 figures. Revised version with erratum for PRA 86, 024302
(2012). Simplified proof that discord is bounded by entanglement in any nor
Limitations to the determination of a Laguerre-Gauss spectrum via projective, phase-flattening measurement
One of the most widely used techniques for measuring the orbital angular
momentum components of a light beam is to flatten the spiral phase front of a
mode, in order to couple it to a single-mode optical fiber. This method,
however, suffers from an efficiency that depends on the orbital angular
momentum of the initial mode and on the presence of higher order radial modes.
The reason is that once the phase has been flattened, the field retains its
ringed intensity pattern and is therefore a nontrivial superposition of purely
radial modes, of which only the fundamental one couples to a single mode
optical fiber. In this paper, we study the efficiency of this technique both
theoretically and experimentally. We find that even for low values of the OAM,
a large amount of light can fall outside the fundamental mode of the fiber, and
we quantify the losses as functions of the waist of the coupling beam of the
orbital angular momentum and radial indices. Our results can be used as a tool
to remove the efficiency bias where fair-sampling loopholes are not a concern.
However, we hope that our study will encourage the development of better
detection methods of the orbital angular momentum content of a beam of light.Comment: 5 pages, 4 figure
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