399 research outputs found
Simple model for 1/f noise
We present a simple stochastic mechanism which generates pulse trains
exhibiting a power law distribution of the pulse intervals and a
power spectrum over several decades at low frequencies with close to
one. The essential ingredient of our model is a fluctuating threshold which
performs a Brownian motion. Whenever an increasing potential hits the
threshold, is reset to the origin and a pulse is emitted. We show that
if increases linearly in time, the pulse intervals can be approximated
by a random walk with multiplicative noise. Our model agrees with recent
experiments in neurobiology and explains the high interpulse interval
variability and the occurrence of noise observed in cortical
neurons and earthquake data.Comment: 4 pages, 4 figure
Geometric approach to nonlinear coherent states using the Higgs model for harmonic oscillator
In this paper, we investigate the relation between the curvature of the
physical space and the deformation function of the deformed oscillator algebra
using non-linear coherent states approach. For this purpose, we study
two-dimensional harmonic oscillators on the flat surface and on a sphere by
applying the Higgs modell. With the use of their algebras, we show that the
two-dimensional oscillator algebra on a surface can be considered as a deformed
one-dimensional oscillator algebra where the effect of the curvature of the
surface is appeared as a deformation function. We also show that the curvature
of the physical space plays the role of deformation parameter. Then we
construct the associated coherent states on the flat surface and on a sphere
and compare their quantum statistical properties, including quadrature
squeezing and antibunching effect.Comment: 12 pages, 7 figs. To be appeared in J. Phys.
From quantum cellular automata to quantum lattice gases
A natural architecture for nanoscale quantum computation is that of a quantum
cellular automaton. Motivated by this observation, in this paper we begin an
investigation of exactly unitary cellular automata. After proving that there
can be no nontrivial, homogeneous, local, unitary, scalar cellular automaton in
one dimension, we weaken the homogeneity condition and show that there are
nontrivial, exactly unitary, partitioning cellular automata. We find a one
parameter family of evolution rules which are best interpreted as those for a
one particle quantum automaton. This model is naturally reformulated as a two
component cellular automaton which we demonstrate to limit to the Dirac
equation. We describe two generalizations of this automaton, the second of
which, to multiple interacting particles, is the correct definition of a
quantum lattice gas.Comment: 22 pages, plain TeX, 9 PostScript figures included with epsf.tex
(ignore the under/overfull \vbox error messages); minor typographical
corrections and journal reference adde
Quantum Langevin equations for semiconductor light-emitting devices and the photon statistics at a low-injection level
From the microscopic quantum Langevin equations (QLEs) we derive the
effective semiconductor QLEs and the associated noise correlations which are
valid at a low-injection level and in real devices. Applying the semiconductor
QLEs to semiconductor light-emitting devices (LEDs), we obtain a new formula
for the Fano factor of photons which gives the photon-number statistics as a
function of the pump statistics and several parameters of LEDs. Key ingredients
are non-radiative processes, carrier-number dependence of the radiative and
non-radiative lifetimes, and multimodeness of LEDs. The formula is applicable
to the actual cases where the quantum efficiency differs from the
differential quantum efficiency , whereas previous theories
implicitly assumed . It is also applicable to the cases when
photons in each mode of the cavity are emitted and/or detected inhomogeneously.
When at a running point, in particular, our formula predicts
that even a Poissonian pump can produce sub-Poissonian light. This mechanism
for generation of sub-Poissonian light is completely different from those of
previous theories, which assumed sub-Poissonian statistics for the current
injected into the active layers of LEDs. Our results agree with recent
experiments. We also discuss frequency dependence of the photon statistics.Comment: 10 pages, 8 figure
One-Way Entangled-Photon Autocompensating Quantum Cryptography
A new quantum cryptography implementation is presented that combines one-way
operation with an autocompensating feature that has hitherto only been
available in implementations that require the signal to make a round trip
between the users. Using the concept of advanced waves, it is shown that this
new implementation is related to the round-trip implementations in the same way
that Ekert's two-particle scheme is related to the original one-particle scheme
of Bennett and Brassard. The practical advantages and disadvantages of the
proposed implementation are discussed in the context of existing schemes.Comment: 5 pages, 1 figure; Minor edits--conclusions unchanged; accepted for
publication in Physical Review
Multi-Parameter Entanglement in Femtosecond Parametric Down-Conversion
A theory of spontaneous parametric down-conversion, which gives rise to a
quantum state that is entangled in multiple parameters, such as
three-dimensional wavevector and polarization, allows us to understand the
unusual characteristics of fourth-order quantum interference in many
experiments, including ultrafast type-II parametric down-conversion, the
specific example illustrated in this paper. The comprehensive approach provided
here permits the engineering of quantum states suitable for quantum information
schemes and new quantum technologies.Comment: to appear in Physical Review
Two-Bit Gates are Universal for Quantum Computation
A proof is given, which relies on the commutator algebra of the unitary Lie
groups, that quantum gates operating on just two bits at a time are sufficient
to construct a general quantum circuit. The best previous result had shown the
universality of three-bit gates, by analogy to the universality of the Toffoli
three-bit gate of classical reversible computing. Two-bit quantum gates may be
implemented by magnetic resonance operations applied to a pair of electronic or
nuclear spins. A ``gearbox quantum computer'' proposed here, based on the
principles of atomic force microscopy, would permit the operation of such
two-bit gates in a physical system with very long phase breaking (i.e., quantum
phase coherence) times. Simpler versions of the gearbox computer could be used
to do experiments on Einstein-Podolsky-Rosen states and related entangled
quantum states.Comment: 21 pages, REVTeX 3.0, two .ps figures available from author upon
reques
Role of entanglement in two-photon imaging
The use of entangled photons in an imaging system can exhibit effects that
cannot be mimicked by any other two-photon source, whatever the strength of the
correlations between the two photons. We consider a two-photon imaging system
in which one photon is used to probe a remote (transmissive or scattering)
object, while the other serves as a reference. We discuss the role of
entanglement versus correlation in such a setting, and demonstrate that
entanglement is a prerequisite for achieving distributed quantum imaging.Comment: 15 pages, 2 figure
Dynamics, correlations and phases of the micromaser
The micromaser possesses a variety of dynamical phase transitions
parametrized by the flux of atoms and the time-of-flight of the atom within the
cavity. We discuss how these phases may be revealed to an observer outside the
cavity using the long-time correlation length in the atomic beam. Some of the
phase transitions are not reflected in the average excitation level of the
outgoing atom, which is the commonly used observable. The correlation length is
directly related to the leading eigenvalue of the time evolution operator,
which we study in order to elucidate the phase structure. We find that as a
function of the time-of-flight the transition from the thermal to the maser
phase is characterized by a sharp peak in the correlation length. For longer
times-of-flight there is a transition to a phase where the correlation length
grows exponentially with the flux. We present a detailed numerical and
analytical treatment of the different phases and discuss the physics behind
them.Comment: 60 pages, 18 figure files, Latex + \special{} for the figures, (some
redundant figures are eliminated and others are changed
Biphoton focusing for two-photon excitation
We study two-photon excitation using biphotons generated via the process of
spontaneous parametric down-conversion in a nonlinear crystal. We show that the
focusing of these biphotons yields an excitation distribution that is
essentially the same as the distribution of one-photon excitation at the pump
wavelength. We also demonstrate that biphoton excitation in the image region
yields a distribution whose axial width is approximately that of the crystal
thickness and whose transverse width is that of the pump at the input to the
crystal.Comment: Accepted for publication in Physical Review
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