984 research outputs found
Balance network of asymmetric simple exclusion process
We investigate a balance network of the asymmetric simple exclusion process
(ASEP). Subsystems consisting of ASEPs are connected by bidirectional links
with each other, which results in balance between every pair of subsystems. The
network includes some specific important cases discussed in earlier works such
as the ASEP with the Langmuir kinetics, multiple lanes and finite reservoirs.
Probability distributions of particles in the steady state are exactly given in
factorized forms according to their balance properties. Although the system has
nonequilibrium parts, the expressions are well described in a framework of
statistical mechanics based on equilibrium states. Moreover, the overall
argument does not depend on the network structures, and the knowledge obtained
in this work is applicable to a broad range of problems
On-chip quantum interference between silicon photon-pair sources
Large-scale integrated quantum photonic technologies1, 2 will require on-chip integration of identical photon sources with reconfigurable waveguide circuits. Relatively complex quantum circuits have been demonstrated already1, 2, 3, 4, 5, 6, 7, but few studies acknowledge the pressing need to integrate photon sources and waveguide circuits together on-chip8, 9. A key step towards such large-scale quantum technologies is the integration of just two individual photon sources within a waveguide circuit, and the demonstration of high-visibility quantum interference between them. Here, we report a silicon-on-insulator device that combines two four-wave mixing sources in an interferometer with a reconfigurable phase shifter. We configured the device to create and manipulate two-colour (non-degenerate) or same-colour (degenerate) path-entangled or path-unentangled photon pairs. We observed up to 100.0 ± 0.4% visibility quantum interference on-chip, and up to 95 ± 4% off-chip. Our device removes the need for external photon sources, provides a path to increasing the complexity of quantum photonic circuits and is a first step towards fully integrated quantum technologies
Exact eigenspectrum of the symmetric simple exclusion process on the complete, complete bipartite, and related graphs
We show that the infinitesimal generator of the symmetric simple exclusion
process, recast as a quantum spin-1/2 ferromagnetic Heisenberg model, can be
solved by elementary techniques on the complete, complete bipartite, and
related multipartite graphs. Some of the resulting infinitesimal generators are
formally identical to homogeneous as well as mixed higher spins models. The
degeneracies of the eigenspectra are described in detail, and the
Clebsch-Gordan machinery needed to deal with arbitrary spin-s representations
of the SU(2) is briefly developed. We mention in passing how our results fit
within the related questions of a ferromagnetic ordering of energy levels and a
conjecture according to which the spectral gaps of the random walk and the
interchange process on finite simple graphs must be equal.Comment: Final version as published, 19 pages, 4 figures, 40 references given
in full forma
Probing the Shape of Quantum Dots with Magnetic Fields
A tool for the identification of the shape of quantum dots is developed. By
preparing a two-electron quantum dot, the response of the low-lying excited
states to a homogeneous magnetic field, i.e. their spin and parity
oscillations, is studied for a large variety of dot shapes. For any geometric
configuration of the confinement we encounter characteristic spin singlet -
triplet crossovers. The magnetization is shown to be a complementary tool for
probing the shape of the dot.Comment: 11 pages, 4 figure
Rotational and vibrational spectra of quantum rings
One can confine the two-dimensional electron gas in semiconductor
heterostructures electrostatically or by etching techniques such that a small
electron island is formed. These man-made ``artificial atoms'' provide the
experimental realization of a text-book example of many-particle physics: a
finite number of quantum particles in a trap. Much effort was spent on making
such "quantum dots" smaller and going from the mesoscopic to the quantum
regime. Far-reaching analogies to the physics of atoms, nuclei or metal
clusters were obvious from the very beginning: The concepts of shell structure
and Hund's rules were found to apply -- just as in real atoms! In this Letter,
we report the discovery that electrons confined in ring-shaped quantum dots
form rather rigid molecules with antiferromagnetic order in the ground state.
This can be seen best from an analysis of the rotational and vibrational
excitations
Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality
We study the energy spectra of small three-dimensional (3D) and
two-dimensional (2D) semiconductor quantum dots through different theoretical
approaches (single-site Hubbard and Hartree-Fock hamiltonians); in the smallest
dots we also compare with exact results. We find that purely 2D models often
lead to an inadequate description of the Coulomb interaction existing in
realistic structures, as a consequence of the overestimated carrier
localization. We show that the dimensionality of the dots has a crucial impact
on (i) the accuracy of the predicted addition spectra; (ii) the range of
validity of approximate theoretical schemes. When applied to realistic 3D
geometries, the latter are found to be much more accurate than in the
corresponding 2D cases for a large class of quantum dots; the single-site
Hubbard hamiltonian is shown to provide a very effective and accurate scheme to
describe quantum dot spectra, leading to good agreement with experiments.Comment: LaTeX 2.09, RevTeX, 25 pages, 9 Encapsulated Postscript figures. To
be published in Physical Review
Full Quantum Analysis of Two-Photon Absorption Using Two-Photon Wavefunction: Comparison with One-Photon Absorption
For dissipation-free photon-photon interaction at the single photon level, we
analyze one-photon transition and two-photon transition induced by photon pairs
in three-level atoms using two-photon wavefunctions. We show that the
two-photon absorption can be substantially enhanced by adjusting the time
correlation of photon pairs. We study two typical cases: Gaussian wavefunction
and rectangular wavefunction. In the latter, we find that under special
conditions one-photon transition is completely suppressed while the high
probability of two-photon transition is maintained.Comment: 6 pages, 4 figure
On the formation of Wigner molecules in small quantum dots
It was recently argued that in small quantum dots the electrons could
crystallize at much higher densities than in the infinite two-dimensional
electron gas. We compare predictions that the onset of spin polarization and
the formation of Wigner molecules occurs at a density parameter to the results of a straight-forward diagonalization of the Hamiltonian
matrix
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