15,579 research outputs found
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
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
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
The Trilinear Hamiltonian: A Zero Dimensional Model of Hawking Radiation from a Quantized Source
We investigate a quantum parametric amplifier with dynamical pump mode,
viewed as a zero-dimensional model of Hawking radiation from an evaporating
black hole. The conditions are derived under which the spectrum of particles
generated from vacuum fluctuations deviates from the thermal spectrum predicted
for the conventional parametric amplifier. We find that significant deviations
arise when the pump mode (black hole) has emitted nearly half of its initial
energy into the signal (Hawking radiation) and idler (in-falling particle)
modes. As a model of black hole dynamics, this finding lends support to the
view that late-time Hawking radiation contains information about the quantum
state of the black hole and is entangled with the black hole's quantum
gravitational degrees of freedom.Comment: 18 pages, 6 figures, Submitted to New Journal of Physics focus issue:
"Classical and Quantum Analogues for Gravitational Phenomena and Related
Effects
The effect of composition on the mechanism of stress-corrosion cracking of titanium alloys in nitrogen tetroxide, and aqueous and hot- salt environments Annual summary report, 1 May 1967 - 30 Apr. 1968
Stress corrosion data for titanium alloys in aqueous, hot salt, and nitrogen dioxide environment
Power-law distributions for the areas of the basins of attraction on a potential energy landscape
Energy landscape approaches have become increasingly popular for analysing a
wide variety of chemical physics phenomena. Basic to many of these applications
has been the inherent structure mapping, which divides up the potential energy
landscape into basins of attraction surrounding the minima. Here, we probe the
nature of this division by introducing a method to compute the basin area
distribution and applying it to some archetypal supercooled liquids. We find
that this probability distribution is a power law over a large number of
decades with the lower-energy minima having larger basins of attraction.
Interestingly, the exponent for this power law is approximately the same as
that for a high-dimensional Apollonian packing, providing further support for
the suggestion that there is a strong analogy between the way the energy
landscape is divided into basins, and the way that space is packed in
self-similar, space-filling hypersphere packings, such as the Apollonian
packing. These results suggest that the basins of attraction provide a
fractal-like tiling of the energy landscape, and that a scale-free pattern of
connections between the minima is a general property of energy landscapes.Comment: 4 pages, 3 figure
Towards Einstein-Podolsky-Rosen quantum channel multiplexing
A single broadband squeezed field constitutes a quantum communication
resource that is sufficient for the realization of a large number N of quantum
channels based on distributed Einstein-Podolsky-Rosen (EPR) entangled states.
Each channel can serve as a resource for, e.g. independent quantum key
distribution or teleportation protocols. N-fold channel multiplexing can be
realized by accessing 2N squeezed modes at different Fourier frequencies. We
report on the experimental implementation of the N=1 case through the
interference of two squeezed states, extracted from a single broadband squeezed
field, and demonstrate all techniques required for multiplexing (N>1). Quantum
channel frequency multiplexing can be used to optimize the exploitation of a
broadband squeezed field in a quantum information task. For instance, it is
useful if the bandwidth of the squeezed field is larger than the bandwidth of
the homodyne detectors. This is currently a typical situation in many
experiments with squeezed and two-mode squeezed entangled light.Comment: 4 pages, 4 figures. In the new version we cite recent experimental
work bei Mehmet et al., arxiv0909.5386, in order to clarify the motivation of
our work and its possible applicatio
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
Comparison of Quantum and Classical Local-field Effects on Two-Level Atoms in a Dielectric
The macroscopic quantum theory of the electromagnetic field in a dielectric
medium interacting with a dense collection of embedded two-level atoms fails to
reproduce a result that is obtained from an application of the classical
Lorentz local-field condition. Specifically, macroscopic quantum
electrodynamics predicts that the Lorentz redshift of the resonance frequency
of the atoms will be enhanced by a factor of the refractive index n of the host
medium. However, an enhancement factor of (n*n+2)/3 is derived using the
Bloembergen procedure in which the classical Lorentz local-field condition is
applied to the optical Bloch equations. Both derivations are short and
uncomplicated and are based on well-established physical theories, yet lead to
contradictory results. Microscopic quantum electrodynamics confirms the
classical local-field-based results. Then the application of macroscopic
quantum electrodynamic theory to embedded atoms is proved false by a specific
example in which both the correspondence principle and microscopic theory of
quantum electrodynamics are violated.Comment: Published version with rewritten abstract and introductio
Applications of Coherent Population Transfer to Quantum Information Processing
We develop a theoretical framework for the exploration of quantum mechanical
coherent population transfer phenomena, with the ultimate goal of constructing
faithful models of devices for classical and quantum information processing
applications. We begin by outlining a general formalism for weak-field quantum
optics in the Schr\"{o}dinger picture, and we include a general
phenomenological representation of Lindblad decoherence mechanisms. We use this
formalism to describe the interaction of a single stationary multilevel atom
with one or more propagating classical or quantum laser fields, and we describe
in detail several manifestations and applications of electromagnetically
induced transparency. In addition to providing a clear description of the
nonlinear optical characteristics of electromagnetically transparent systems
that lead to ``ultraslow light,'' we verify that -- in principle -- a
multi-particle atomic or molecular system could be used as either a low power
optical switch or a quantum phase shifter. However, we demonstrate that the
presence of significant dephasing effects destroys the induced transparency,
and that increasing the number of particles weakly interacting with the probe
field only reduces the nonlinearity further. Finally, a detailed calculation of
the relative quantum phase induced by a system of atoms on a superposition of
spatially distinct Fock states predicts that a significant quasi-Kerr
nonlinearity and a low entropy cannot be simultaneously achieved in the
presence of arbitrary spontaneous emission rates. Within our model, we identify
the constraints that need to be met for this system to act as a one-qubit and a
two-qubit conditional phase gate.Comment: 25 pages, 14 figure
- …