7,696 research outputs found
Correlations in a BEC collision: First-principles quantum dynamics with 150 000 atoms
The quantum dynamics of colliding Bose-Einstein condensates with 150 000
atoms are simulated directly from the Hamiltonian using the stochastic
positive-P method. Two-body correlations between the scattered atoms and their
velocity distribution are found for experimentally accessible parameters.
Hanbury Brown-Twiss or thermal-like correlations are seen for copropagating
atoms, while number correlations for counterpropagating atoms are even stronger
than thermal correlations at short times. The coherent phase grains grow in
size as the collision progresses with the onset of growth coinciding with the
beginning of stimulated scattering. The method is versatile and usable for a
range of cold atom systems.Comment: 4 pages, 4 figures. v2: Rewording and style changes, minor except for
rewrite of background on the positive-P representation. Original research
unchange
Many-body quantum dynamics of polarisation squeezing in optical fibre
We report new experiments that test quantum dynamical predictions of
polarization squeezing for ultrashort photonic pulses in a birefringent fibre,
including all relevant dissipative effects. This exponentially complex
many-body problem is solved by means of a stochastic phase-space method. The
squeezing is calculated and compared to experimental data, resulting in
excellent quantitative agreement. From the simulations, we identify the
physical limits to quantum noise reduction in optical fibres. The research
represents a significant experimental test of first-principles time-domain
quantum dynamics in a one-dimensional interacting Bose gas coupled to
dissipative reservoirs.Comment: 4 pages, 4 figure
Quantum theory of dispersive electromagnetic modes
A quantum theory of dispersion for an inhomogeneous solid is obtained, from a
starting point of multipolar coupled atoms interacting with an electromagnetic
field. The dispersion relations obtained are equivalent to the standard
classical Sellmeir equations obtained from the Drude-Lorentz model. In the
homogeneous (plane-wave) case, we obtain the detailed quantum mode structure of
the coupled polariton fields, and show that the mode expansion in all branches
of the dispersion relation is completely defined by the refractive index and
the group-velocity for the polaritons. We demonstrate a straightforward
procedure for exactly diagonalizing the Hamiltonian in one, two or
three-dimensional environments, even in the presence of longitudinal
phonon-exciton dispersion, and an arbitrary number of resonant transitions with
different frequencies. This is essential, since it is necessary to include at
least one phonon (I.R.) and one exciton (U.V.) mode, in order to accurately
represent dispersion in transparent solid media. Our method of diagonalization
does not require an explicit solution of the dispersion relation, but relies
instead on the analytic properties of Cauchy contour integrals over all
possible mode frequencies. When there is longitudinal phonon dispersion, the
relevant group-velocity term is modified so that it only includes the purely
electromagnetic part of the group velocity
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
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
Manipulating Majorana fermions in one-dimensional spin-orbit coupled atomic Fermi gases
Majorana fermions are promising candidates for storing and processing
information in topological quantum computation. The ability to control such
individual information carriers in trapped ultracold atomic Fermi gases is a
novel theme in quantum information science. However, fermionic atoms are
neutral and thus are difficult to manipulate. Here, we theoretically
investigate the control of emergent Majorana fermions in one-dimensional
spin-orbit coupled atomic Fermi gases. We discuss (i) how to move Majorana
fermions by increasing or decreasing an effective Zeeman field, which acts like
a solid state control voltage gate; and (ii) how to create a pair of Majorana
fermions by adding a magnetic impurity potential. We discuss the experimental
realization of our control scheme in an ultracold Fermi gas of K atoms.Comment: 4 papges, 6 figure
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
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
Naturally-phasematched second harmonic generation in a whispering gallery mode resonator
We demonstrate for the first time natural phase matching for optical
frequency doubling in a high-Q whispering gallery mode resonator made of
Lithium Niobate. A conversion efficiency of 9% is achieved at 30 micro Watt
in-coupled continuous wave pump power. The observed saturation pump power of
3.2 mW is almost two orders of magnitude lower than the state-of-the-art. This
suggests an application of our frequency doubler as a source of non-classical
light requiring only a low-power pump, which easily can be quantum noise
limited. Our theoretical analysis of the three-wave mixing in a whispering
gallery mode resonator provides the relative conversion efficiencies for
frequency doubling in various modes
- …