10,241 research outputs found
A Langevin analysis of fundamental noise limits in Coherent Anti-Stokes Raman Spectroscopy
We use a Langevin approach to analyze the quantum noise in Coherent
Anti-Stokes Raman Spectroscopy (CARS) in several experimental scenarios: with
continuous wave input fields acting simultaneously and with fast sequential
pulsed lasers where one field scatters off the coherence generated by other
fields; and for interactions within a cavity and in free space. In all the
cases, the signal as well as the quantum noise due to spontaneous decay and
decoherence in the medium are shown to be described by the same general
expression. Our theory in particular shows that for short interaction times,
the medium noise is not important and the efficiency is limited only by the
intrinsic quantum nature of the photon. We obtain fully analytic results
\emph{without} making an adiabatic approximation, the fluctuations of the
medium and the fields are self solved consistently.Comment: 12 pages, 1 figur
Electron transport through interacting quantum dots
We present a detailed theoretical investigation of the effect of Coulomb
interactions on electron transport through quantum dots and double barrier
structures connected to a voltage source via an arbitrary linear impedance.
Combining real time path integral techniques with the scattering matrix
approach we derive the effective action and evaluate the current-voltage
characteristics of quantum dots at sufficiently large conductances. Our
analysis reveals a reach variety of different regimes which we specify in
details for the case of chaotic quantum dots. At sufficiently low energies the
interaction correction to the current depends logarithmically on temperature
and voltage. We identify two different logarithmic regimes with the crossover
between them occurring at energies of order of the inverse dwell time of
electrons in the dot. We also analyze the frequency-dependent shot noise in
chaotic quantum dots and elucidate its direct relation to interaction effects
in mesoscopic electron transport.Comment: 21 pages, 4 figures. References added, discussion slightly extende
Laser theory in manifest Lindblad form
We discuss the laser theory for a single-mode laser with nonlinear gain. We
focus in particular on a micromaser which is pumped with a dilute beam of
excited atoms crossing the laser cavity. In the weak-coupling regime, an
expansion in the coupling strength is developed that preserves the Lindblad
form of the master equation, securing the positivity of the density matrix.
This expansion breaks rapidly down above threshold. This can be improved with
an alternative approach, not restricted to weak coupling: the Lindblad
operators are expanded in orthogonal polynomials adapted to the probability
distribution for the atom-laser interaction time. Results for the photon
statistics and the laser linewidth illustrate the theory.Comment: 13 pages, 8 figures, to be published in J Phys B (minor revision
Persistent current noise and electron-electron interactions
We analyze fluctuations of persistent current (PC) produced by a charged
quantum particle moving in a ring and interacting with a dissipative
environment formed by diffusive electron gas. We demonstrate that in the
presence of interactions such PC fluctuations persist down to zero temperature.
In the case of weak interactions and/or sufficiently small values of the ring
radius PC noise remains coherent and can be tuned by external magnetic flux
piercing the ring. In the opposite limit of strong interactions and/or
large values of fluctuations in the electronic bath strongly suppress
quantum coherence of the particle down to and induce incoherent
-independent current noise in the ring which persists even at
when the average PC is absent.Comment: 12 pages, 8 figure
Coulomb Interaction and Quantum Transport through a Coherent Scatterer
An interplay between charge discreteness, coherent scattering and Coulomb
interaction yields nontrivial effects in quantum transport. We derive a real
time effective action and an equivalent quantum Langevin equation for an
arbitrary coherent scatterer and evaluate its current-voltage characteristics
in the presence of interactions. Within our model, at large conductances
and low (but outside the instanton-dominated regime) the interaction
correction to saturates and causes conductance suppression by a universal
factor which depends only on the type of the conductor.Comment: 4 pages, no figure
Nonequilibrium phenomena in multiple normal-superconducting tunnel heterostructures
Using the nonequilibrium theory of superconductivity with the tunnel
Hamiltonian, we consider a mesoscopic NISINISIN heterostructure, i.e., a
structure consisting of five intermittent normal-metal (N) and superconducting
(S) regions separated by insulating tunnel barriers (I). Applying the bias
voltage between the outer normal electrodes one can drive the central N island
very far from equilibrium. Depending on the resistance ratio of outer and inner
tunnel junctions, one can realize either effective electron cooling in the
central N island or create highly nonequilibrium energy distributions of
electrons in both S and N islands. These distributions exhibit multiple peaks
at a distance of integer multiples of the superconducting chemical potential.
In the latter case the superconducting gap in the S islands is strongly
suppressed as compared to its equilibrium value
Electron transport and current fluctuations in short coherent conductors
Employing a real time effective action formalism we analyze electron
transport and current fluctuations in comparatively short coherent conductors
in the presence of electron-electron interactions. We demonstrate that, while
Coulomb interaction tends to suppress electron transport, it may {\it strongly
enhance} shot noise in scatterers with highly transparent conducting channels.
This effect of excess noise is governed by the Coulomb gap observed in the
current-voltage characteristics of such scatterers. We also analyze the
frequency dispersion of higher current cumulants and emphasize a direct
relation between electron-electron interaction effects and current fluctuations
in disordered mesoscopic conductors.Comment: 16 pages, 4 figure
Parity-Affected Superconductivity in Ultrasmall Metallic Grains
We investigate the breakdown of BCS superconductivity in {\em ultra}\/small
metallic grains as a function of particle size (characterized by the mean
spacing between discrete electronic eigenstates), and the parity ( =
even/odd) of the number of electrons on the island. Assuming equally spaced
levels, we solve the parity-dependent BCS gap equation for the order parameter
. Both the critical level spacing and the
critical temperature at which are parity
dependent, and both are so much smaller in the odd than the even case that
these differences should be measurable in current experiments.Comment: 4 pages RevTeX, 1 encapsulated postscript figure, submitted to
Physical Review Letter
Semiclassical kinetic theory of electron spin relaxation in semiconductors
We develop a semiclassical kinetic theory for electron spin relaxation in
semiconductors. Our approach accounts for elastic as well as inelastic
scattering and treats Elliott-Yafet and motional-narrowing processes, such as
D'yakonov-Perel' and variable g-factor processes, on an equal footing. Focusing
on small spin polarizations and small momentum transfer scattering, we derive,
starting from the full quantum kinetic equations, a Fokker-Planck equation for
the electron spin polarization. We then construct, using a rigorous multiple
time scale approach, a Bloch equation for the macroscopic (-averaged)
spin polarization on the long time scale, where the spin polarization decays.
Spin-conserving energy relaxation and diffusion, which occur on a fast time
scale, after the initial spin polarization has been injected, are incorporated
and shown to give rise to a weight function which defines the energy averages
required for the calculation of the spin relaxation tensor in the Bloch
equation. Our approach provides an intuitive way to conceptualize the dynamics
of the spin polarization in terms of a ``test'' spin polarization which
scatters off ``field'' particles (electrons, impurities, phonons). To
illustrate our approach, we calculate for a quantum well the spin lifetime at
temperatures and densities where electron-electron and electron-impurity
scattering dominate. The spin lifetimes are non-monotonic functions of
temperature and density. Our results show that at electron densities and
temperatures, where the cross-over from the non-degenerate to the degenerate
regime occurs, spin lifetimes are particularly long.Comment: 29 pages, 10 figures, final versio
Dephasing Times in a Non-degenerate Two-Dimensional Electron Gas
Studies of weak localization by scattering from vapor atoms for electrons on
a liquid helium surface are reported. There are three contributions to the
dephasing time. Dephasing by the motion of vapor atoms perpendicular to the
surface is studied by varying the holding field to change the characteristic
width of the electron layer at the surface. A change in vapor density alters
the quasi-elastic scattering length and the dephasing due to the motion of
atoms both perpendicular and parallel to the surface. Dephasing due to the
electron-electron interaction is dependent on the electron density.Comment: 4 pages, Revte
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