126 research outputs found
Detecting entanglement using a double quantum dot turnstile
We propose a scheme based on using the singlet ground state of an electron
spin pair in a double quantum dot nanostructure as a suitable set-up for
detecting entanglement between electron spins via the measurement of an optimal
entanglement witness. Using time-dependent gate voltages and magnetic fields
the entangled spins are separated and coherently rotated in the quantum dots
and subsequently detected at spin-polarized quantum point contacts. We analyze
the coherent time evolution of the entangled pair and show that by counting
coincidences in the four exits an entanglement test can be done. This set-up is
close to present-day experimental possibilities and can be used to produce
pairs of entangled electrons ``on demand''.Comment: 5 pages, 2 figures - published versio
Quantum Pumping and Quantized Magnetoresistance in a Hall Bar
We show how a dc current can be generated in a Hall bar without applying a
bias voltage. The Hall resistance that corresponds to this pumped current
is quantized, just as in the usual integer quantum Hall effect (IQHE). In
contrast with the IQHE, however, the longitudinal resistance does not
vanish on the plateaus, but equals the Hall resistance. We propose an
experimental geometry to measure the pumped current and verify the predicted
behavior of and .Comment: RevTeX, 3 figure
Spatiotemporally localized solitons in resonantly absorbing Bragg reflectors
We predict the existence of spatiotemporal solitons (``light bullets'') in
two-dimensional self-induced transparency media embedded in a Bragg grating.
The "bullets" are found in an approximate analytical form, their stability
being confirmed by direct simulations. These findings suggest new possibilities
for signal transmission control and self-trapping of light.Comment: RevTex, 3 pages, 2 figures, to be published in PR
Resonance approximation and charge loading/unloading in adiabatic quantum pumping
Quantum pumping through mesoscopic quantum dots is known to be enhanced by
resonant transmission. The pumped charge is close to an integer number of
electrons when the pumping contour surrounds a resonance, but the transmission
remains small on the contour. For non-interacting electrons, we give a
quantitative account of the detailed exchange of electrons between the dot and
the leads (to the electron reservoirs) during a pumping cycle. Near isolated
distinct resonances, we use approximate Breit-Wigner expressions for the dot's
Green function to discuss the loading/unloading picture of the pumping: the
fractional charge exchanged between the dot and each lead through a single
resonance point is related to the relative couplings of the dot and the leads
at this resonance. If each resonance point along the pumping contour is
dominated by the coupling to a single lead (which also implies a very small
transmission), then the crossing of each such resonance results in a single
electron exchange between the dot and that lead, ending up with a net quantized
charge. When the resonance approximation is valid, the fractional charges can
also be extracted from the peaks of the transmissions between the various
leads.Comment: 10 pages, 4 figure
Helicity Modulus and Effective Hopping in the Two-Dimensional Hubbard Model Using Slave-Boson Methods
The slave-boson mean-field method is used to study the two-dimensional
Hubbard model. A magnetic phase diagram allowing for paramagnetism, weak- and
strong ferromagnetism and antiferromagnetism, including all continuous and
first-order transitions, is constructed and compared to the corresponding phase
diagram using the Hartree-Fock approximation (HFA). Magnetically ordered
regions are reduced by a factor of about 3 along both the and density
axes compared to the HFA. Using the spin-rotation invariant formulation of the
slave-boson method the helicity modulus is computed and for half-filling is
found to practically coincide with that found using variational Monte Carlo
calculations using the Gutzwiller wave function. Off half-filling the results
can be used to compare with Quantum Monte Carlo calculations of the effective
hopping parameter. Contrary to the case of half-filling, the slave-boson
approach is seen to greatly improve the results of the HFA when off
half-filling. (Submitted to: Journal of Physics: Condensed Matter)Comment: 27 pages, LaTeX2e, 7 figures available upon request, INLO-PUB-10/9
Dynamic effect of phase conjugation on wave localization
We investigate what would happen to the time dependence of a pulse reflected
by a disordered single-mode waveguide, if it is closed at one end not by an
ordinary mirror but by a phase-conjugating mirror. We find that the waveguide
acts like a virtual cavity with resonance frequency equal to the working
frequency omega_0 of the phase-conjugating mirror. The decay in time of the
average power spectrum of the reflected pulse is delayed for frequencies near
omega_0. In the presence of localization the resonance width is
tau_s^{-1}exp(-L/l), with L the length of the waveguide, l the mean free path,
and tau_s the scattering time. Inside this frequency range the decay of the
average power spectrum is delayed up to times t simeq tau_s exp(L/l).Comment: 10 pages including 2 figure
Spatiotemporally Localized Multidimensional Solitons in Self-Induced Transparency Media
"Light bullets" are multi-dimensional solitons which are localized in both
space and time. We show that such solitons exist in two- and three-dimensional
self-induced-transparency media and that they are fully stable. Our approximate
analytical calculation, backed and verified by direct numerical simulations,
yields the multi-dimensional generalization of the one-dimensional Sine-Gordon
soliton.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Andreev Bound States and Self-Consistent Gap Functions for SNS and SNSNS Systems
Andreev bound states in clean, ballistic SNS and SNSNS junctions are
calculated exactly and by using the Andreev approximation (AA). The AA appears
to break down for junctions with transverse dimensions chosen such that the
motion in the longitudinal direction is very slow. The doubly degenerate states
typical for the traveling waves found in the AA are replaced by two standing
waves in the exact treatment and the degeneracy is lifted.
A multiple-scattering Green's function formalism is used, from which the
states are found through the local density of states. The scattering by the
interfaces in any layered system of ballistic normal metals and clean
superconducting materials is taken into account exactly. The formalism allows,
in addition, for a self-consistent determination of the gap function. In the
numerical calculations the pairing coupling constant for aluminum is used.
Various features of the proximity effect are shown
Random Scattering by Atomic Density Fluctuations in Optical Lattices
We investigate hitherto unexplored regimes of probe scattering by atoms
trapped in optical lattices: weak scattering by effectively random atomic
density distributions and multiple scattering by arbitrary atomic
distributions. Both regimes are predicted to exhibit a universal semicircular
scattering lineshape for large density fluctuations, which depend on
temperature and quantum statistics.Comment: 4 pages, 2 figure
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