73,593 research outputs found
Entanglement by linear SU(2) transformations: generation and evolution of quantum vortex states
We consider the evolution of a two-mode system of bosons under the action of
a Hamiltonian that generates linear SU(2) transformations. The Hamiltonian is
generic in that it represents a host of entanglement mechanisms, which can thus
be treated in a unified way. We start by solving the quantum dynamics
analytically when the system is initially in a Fock state. We show how the two
modes get entangled by evolution to produce a coherent superposition of vortex
states in general, and a single vortex state under certain conditions. The
degree of entanglement between the modes is measured by finding the explicit
analytical dependence of the Von Neumann entropy on the system parameters. The
reduced state of each mode is analyzed by means of its correlation function and
spatial coherence function. Remarkably, our analysis is shown to be equally as
valid for a variety of initial states that can be prepared from a two-mode Fock
state via a unitary transformation and for which the results can be obtained by
mere inspection of the corresponding results for an initial Fock state. As an
example, we consider a quantum vortex as the initial state and also find
conditions for its revival and charge conjugation. While studying the evolution
of the initial vortex state, we have encountered and explained an interesting
situation in which the entropy of the system does not evolve whereas its wave
function does. Although the modal concept has been used throughout the paper,
it is important to note that the theory is equally applicable for a
two-particle system in which each particle is represented by its bosonic
creation and annihilation operators.Comment: 6 figure
Nonclassicality and decoherence of photon-subtracted squeezed states
We discuss nonclassical properties of single-photon subtracted squeezed
vacuum states in terms of the sub-Poissonian statistics and the negativity of
the Wigner function. We derive a compact expression for the Wigner function
from which we find the region of phase space where Wigner function is negative.
We find an upper bound on the squeezing parameter for the state to exhibit
sub-Poissonian statistics. We then study the effect of decoherence on the
single-photon subtracted squeezed states. We present results for two different
models of decoherence, viz. amplitude decay model and the phase diffusion
model. In each case we give analytical results for the time evolution of the
state. We discuss the loss of nonclassicality as a result of decoherence. We
show through the study of their phase-space properties how these states decay
to vacuum due to the decay of photons. We show that phase damping leads to very
slow decoherence than the photon-number decay.Comment: Figures are in GIF format separately, submitte
Determination of the Wigner function from photon statistics
We present an experimental realisation of the direct scheme for measuring the
Wigner function of a single quantized light mode. In this method, the Wigner
function is determined as the expectation value of the photon number parity
operator for the phase space displaced quantum state.Comment: 4 pages LaTeX, contribution to proceedings of 6th central-european
workshop on quantum optics; see also
http://www.fuw.edu.pl/~kbanasz/QOLab/ExpWigner
Transfer of an unknown quantum state, quantum networks, and memory
We present a protocol for transfer of an unknown quantum state. The protocol
is based on a two-mode cavity interacting dispersively in a sequential manner
with three-level atoms in configuration. We propose a scheme for
quantum networking using an atomic channel. We investigate the effect of cavity
decoherence in the entire process. Further, we demonstrate the possibility of
an efficient quantum memory for arbitrary superposition of two modes of a
cavity contaning one photon.Comment: 5 pages, 4 figures, RevTeX4, Submitted to PR
Towards Heisenberg Limit in Magnetometry with Parametric Down Converted Photons
Recent theoretical and experimental papers have shown how one can achieve
Heisenberg limited measurements by using entangled photons. Here we show how
the photons in non-collinear down conversion process can be used for improving
the sensitivity of magneto-optical rotation by a factor of four which takes us
towards the Heisenberg limit. Our results apply to sources with arbitrary
pumping. We also present several generalizations of earlier results for the
collinear geometry. The sensitivity depends on whether the two-photon or
four-photon coincidence detection is used.Comment: 4.2 pages, 6 figure
Plane curves with small linear orbits I
The `linear orbit' of a plane curve of degree d is its orbit in the
projective space of dimension d(d+3)/2 parametrizing such curves under the
natural action of PGL(3). In this paper we compute the degree of the closure of
the linear orbits of most curves with positive dimensional stabilizers. Our
tool is a nonsingular variety dominating the orbit closure, which we construct
by a blow-up sequence mirroring the sequence yielding an embedded resolution of
the curve.
The results given here will serve as an ingredient in the computation of the
analogous information for arbitrary plane curves. Linear orbits of smooth plane
curves are studied in [A-F1].Comment: 34 pages, 4 figures, AmS-TeX 2.1, requires xy-pic and eps
Photon-Photon Correlations as a Probe of Vacuum Induced Coherence Effects
We present new experimental implications of the effects of vacuum induced
coherence on the photon -photon correlation in the pi-polarized fluorescence in
j = 1/2 to j = 1/2 transition. These effects should be thus observable in
measurements of photon statistics in for example Hg and Ba ion traps.Comment: 7 pages, 6 figures, submitted to Physical Review
Sub and Super-Luminal Propagation of Intense Pulses in Media with Saturated and Reverse Absorption
We develop models for the propagation of intense pulses in solid state media
which can have either saturated absorption or exhibit reverse absorption . We
show that the experiments of Bigelow {\it et al.}[Phys. Rev. Lett. {\bf 90},
113903 (2003); Science {\bf 301}, 200 (2003).] on subluminal propagation in
Ruby and superluminal propagation in Alexandrite are well explained by
modelling them as three level and four level systems coupled to Maxwell
equations. We present results well beyond the traditional pump-probe approach.Comment: 4 pages, 6 figure
Quantum logic gates using Stark shifted Raman transitions in a cavity
We present a scheme to realise the basic two-quibit logic gates such as
quantum phase gate and controlle-NOT gate using a detuned optical cavity
interacting with a three-level Raman system. We discuss the role of Stark
shifts which are as important as the terms leading to two-photon transition.
The operation of the proposed logic gates involves metastable states of the
atom and hence is not affected by spontaneous emission. These ideas can be
extended to produce multiparticle entanglement.Comment: 5 pages, 1 figure, RevTeX4, Text is modifie
Schottky-based band lineups for refractory semiconductors
An overview is presented of band alignments for small-lattice parameter, refractory semiconductors. The band alignments are estimated empirically through the use of available Schottky barrier height data, and are compared to theoretically predicted values. Results for tetrahedrally bonded semiconductors with lattice constant values in the range from C through ZnSe are presented. Based on the estimated band alignments and the recently demonstrated p-type dopability of GaN, we propose three novel heterojunction schemes which seek to address inherent difficulties in doping or electrical contact to wide-gap semiconductors such as ZnO, ZnSe, and ZnS
- …