28,189 research outputs found
All-Optical Depletion of Dark Excitons from a Semiconductor Quantum Dot
Semiconductor quantum dots are considered to be the leading venue for
fabricating on-demand sources of single photons. However, the generation of
long-lived dark excitons imposes significant limits on the efficiency of these
sources. We demonstrate a technique that optically pumps the dark exciton
population and converts it to a bright exciton population, using intermediate
excited biexciton states. We show experimentally that our method considerably
reduces the DE population while doubling the triggered bright exciton emission,
approaching thereby near-unit fidelity of quantum dot depletion.Comment: 5 pages, 3 figure
Lexicographic choice functions without archimedeanicity
We investigate the connection between choice functions and lexicographic probabilities, by means of the convexity axiom considered by Seidenfeld, Schervisch and Kadane (2010) but without imposing any Archimedean condition. We show that lexicographic probabilities are related to a particular type of sets of desirable gambles, and investigate the properties of the coherent choice function this induces via maximality. Finally, we show that the convexity axiom is necessary but not sufficient for a coherent choice function to be the infimum of a class of lexicographic ones
Development of high critical current density in multifilamentary round-wire Bi2Sr2CaCu2O8+x by strong overdoping
Bi2Sr2CaCu2O8+x is the only cuprate superconductor that can be made into a
round-wire conductor form with a high enough critical current density Jc for
applications. Here we show that the Jc(5 T,4.2 K) of such Ag-sheathed
filamentary wires can be doubled to more than 1.4x10^5 A/cm^2 by low
temperature oxygenation. Careful analysis shows that the improved performance
is associated with a 12 K reduction in transition temperature Tc to 80 K and a
significant enhancement in intergranular connectivity. In spite of the
macroscopically untextured nature of the wire, overdoping is highly effective
in producing high Jc values.Comment: 4 figure
Hall effect in quasi one-dimensional organic conductors
We study the Hall effect in a system of weakly coupled Luttinger Liquid
chains, using a Memory function approach to compute the Hall constant in the
presence of umklapp scattering along the chains. In this approximation, the
Hall constant decomposes into two terms: a high-frequency term and a Memory
function term. For the case of zero umklapp scattering, where the Memory
function vanishes, the Hall constant is simply the band value, in agreement
with former results in a similar model with no dissipation along the chains.
With umklapp scattering along the chains, we find a power-law temperature
dependance of the Hall constant. We discuss the applications to quasi 1D
organic conductors at high temperatures.Comment: Proceedings of the ISCOM conference "Sixth International Symposium on
Crystalline Organic Metals, Superconductors, and Ferromagnets", Key West,
Florida, USA (Sept. 2005), to be plublished in the Journal of Low Temperature
Physic
Complete control of a matter qubit using a single picosecond laser pulse
We demonstrate for the first time that a matter physical two level system, a
qubit, can be fully controlled using one ultrafast step. We show that the spin
state of an optically excited electron, an exciton, confined in a quantum dot,
can be rotated by any desired angle, about any desired axis, during such a
step. For this we use a single, resonantly tuned, picosecond long, polarized
optical pulse. The polarization of the pulse defines the rotation axis, while
the pulse detuning from a non-degenerate absorption resonance, defines the
magnitude of the rotation angle. We thereby achieve a high fidelity, universal
gate operation, applicable to other spin systems, using only one short optical
pulse. The operation duration equals the pulse temporal width, orders of
magnitude shorter than the qubit evolution life and coherence times.Comment: main text: 4 pages, 3 figures Supplemental material: 3 pages, 1
figur
Coherent tunneling by adiabatic passage in an optical waveguide system
We report on the first experimental demonstration of light transfer in an
engineered triple-well optical waveguide structure which provides a classic
analogue of Coherent Tunnelling by Adiabatic Passage (CTAP) recently proposed
for coherent transport in space of neutral atoms or electrons among
tunneling-coupled optical traps or quantum wells [A.D. Greentree et al., Phys.
Rev. B 70, 235317 (2004); K. Eckert et al., Phys. Rev. A 70, 023606 (2004)].
The direct visualization of CTAP wavepacket dynamics enabled by our simple
optical system clearly shows that in the counterintuitive passage scheme light
waves tunnel between the two outer wells without appreciable excitation of the
middle well.Comment: submitted for publicatio
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
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