9,505 research outputs found
Gaussian operator bases for correlated fermions
We formulate a general multi-mode Gaussian operator basis for fermions, to
enable a positive phase-space representation of correlated Fermi states. The
Gaussian basis extends existing bosonic phase-space methods to Fermi systems
and thus enables first-principles dynamical or equilibrium calculations in
quantum many-body Fermi systems. We prove the completeness and positivity of
the basis, and derive differential forms for products with one- and two-body
operators. Because the basis satisfies fermionic superselection rules, the
resulting phase space involves only c-numbers, without requiring anti-commuting
Grassmann variables
Quantum noise in optical fibers II: Raman jitter in soliton communications
The dynamics of a soliton propagating in a single-mode optical fiber with
gain, loss, and Raman coupling to thermal phonons is analyzed. Using both
soliton perturbation theory and exact numerical techniques, we predict that
intrinsic thermal quantum noise from the phonon reservoirs is a larger source
of jitter and other perturbations than the gain-related Gordon-Haus noise, for
short pulses, assuming typical fiber parameters. The size of the Raman timing
jitter is evaluated for both bright and dark (topological) solitons, and is
larger for bright solitons. Because Raman thermal quantum noise is a nonlinear,
multiplicative noise source, these effects are stronger for the more intense
pulses needed to propagate as solitons in the short-pulse regime. Thus Raman
noise may place additional limitations on fiber-optical communications and
networking using ultrafast (subpicosecond) pulses.Comment: 3 figure
Diffusion quantum Monte Carlo calculation of the quasiparticle effective mass of the two-dimensional homogeneous electron gas
The quasiparticle effective mass is a key quantity in the physics of electron
gases, describing the renormalization of the electron mass due to
electron-electron interactions. Two-dimensional electron gases are of
fundamental importance in semiconductor physics, and there have been numerous
experimental and theoretical attempts to determine the quasiparticle effective
mass in these systems. In this work we report quantum Monte Carlo results for
the quasiparticle effective mass of a two-dimensional homogeneous electron gas.
Our calculations differ from previous quantum Monte Carlo work in that much
smaller statistical error bars have been achieved, allowing for an improved
treatment of finite-size effects. In some cases we have also been able to use
larger system sizes than previous calculations
Quantum Monte Carlo calculation of the energy band and quasiparticle effective mass of the two-dimensional Fermi fluid
We have used the diffusion quantum Monte Carlo method to calculate the energy
band of the two-dimensional homogeneous electron gas (HEG), and hence we have
obtained the quasiparticle effective mass and the occupied bandwidth. We find
that the effective mass in the paramagnetic HEG increases significantly when
the density is lowered, whereas it decreases in the fully ferromagnetic HEG.
Our calculations therefore support the conclusions of recent experimental
studies [Y.-W. Tan et al., Phys. Rev. Lett. 94, 016405 (2005); M. Padmanabhan
et al., Phys. Rev. Lett. 101, 026402 (2008); T. Gokmen et al., Phys. Rev. B 79,
195311 (2009)]. We compare our calculated effective masses with other
theoretical results and experimental measurements in the literature
Quantum noise in optical fibers I: stochastic equations
We analyze the quantum dynamics of radiation propagating in a single mode
optical fiber with dispersion, nonlinearity, and Raman coupling to thermal
phonons. We start from a fundamental Hamiltonian that includes the principal
known nonlinear effects and quantum noise sources, including linear gain and
loss. Both Markovian and frequency-dependent, non-Markovian reservoirs are
treated. This allows quantum Langevin equations to be calculated, which have a
classical form except for additional quantum noise terms. In practical
calculations, it is more useful to transform to Wigner or +
quasi-probability operator representations. These result in stochastic
equations that can be analyzed using perturbation theory or exact numerical
techniques. The results have applications to fiber optics communications,
networking, and sensor technology.Comment: 1 figur
Quantum Monte Carlo study of the ground state of the two-dimensional Fermi fluid
We have used the variational and diffusion quantum Monte Carlo methods to
calculate the energy, pair correlation function, static structure factor, and
momentum density of the ground state of the two-dimensional homogeneous
electron gas. We have used highly accurate Slater-Jastrow-backflow trial wave
functions and twist averaging to reduce finite-size effects where applicable.
We compare our results with others in the literature and construct a
local-density-approximation exchange-correlation functional for 2D systems
Gaussian quantum Monte Carlo methods for fermions
We introduce a new class of quantum Monte Carlo methods, based on a Gaussian
quantum operator representation of fermionic states. The methods enable
first-principles dynamical or equilibrium calculations in many-body Fermi
systems, and, combined with the existing Gaussian representation for bosons,
provide a unified method of simulating Bose-Fermi systems. As an application,
we calculate finite-temperature properties of the two dimensional Hubbard
model.Comment: 4 pages, 3 figures, Revised version has expanded discussion,
simplified mathematical presentation, and application to 2D Hubbard mode
Disagreement between correlations of quantum mechanics and stochastic electrodynamics in the damped parametric oscillator
Intracavity and external third order correlations in the damped nondegenerate
parametric oscillator are calculated for quantum mechanics and stochastic
electrodynamics (SED), a semiclassical theory. The two theories yield greatly
different results, with the correlations of quantum mechanics being cubic in
the system's nonlinear coupling constant and those of SED being linear in the
same constant. In particular, differences between the two theories are present
in at least a mesoscopic regime. They also exist when realistic damping is
included. Such differences illustrate distinctions between quantum mechanics
and a hidden variable theory for continuous variables.Comment: accepted by PR
First-principles quantum dynamics in interacting Bose gases I: The positive P representation
The performance of the positive P phase-space representation for exact
many-body quantum dynamics is investigated. Gases of interacting bosons are
considered, where the full quantum equations to simulate are of a
Gross-Pitaevskii form with added Gaussian noise. This method gives tractable
simulations of many-body systems because the number of variables scales
linearly with the spatial lattice size. An expression for the useful simulation
time is obtained, and checked in numerical simulations. The dynamics of first-,
second- and third-order spatial correlations are calculated for a uniform
interacting 1D Bose gas subjected to a change in scattering length. Propagation
of correlations is seen. A comparison is made to other recent methods. The
positive P method is particularly well suited to open systems as no
conservation laws are hard-wired into the calculation. It also differs from
most other recent approaches in that there is no truncation of any kind.Comment: 21 pages, 7 figures, 2 tables, IOP styl
Exciton and biexciton energies in bilayer systems
We report calculations of the energies of excitons and biexcitons in ideal
two-dimensional bilayer systems within the effective-mass approximation with
isotropic electron and hole masses. The exciton energies are obtained by a
simple numerical integration technique, while the biexciton energies are
obtained from diffusion quantum Monte Carlo calculations. The exciton binding
energy decays as the inverse of the separation of the layers, while the binding
energy of the biexciton with respect to dissociation into two separate excitons
decays exponentially
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