1,952 research outputs found
Hide-and-Seek with Directional Sensing
We consider a game played between a hider, who hides a static object in one
of several possible positions in a bounded planar region, and a searcher, who
wishes to reach the object by querying sensors placed in the plane. The
searcher is a mobile agent, and whenever it physically visits a sensor, the
sensor returns a random direction, corresponding to a half-plane in which the
hidden object is located. We first present a novel search heuristic and
characterize bounds on the expected distance covered before reaching the
object. Next, we model this game as a large-dimensional zero-sum dynamic game
and we apply a recently introduced randomized sampling technique that provides
a probabilistic level of security to the hider. We observe that, when the
randomized sampling approach is only allowed to select a very small number of
samples, the cost of the heuristic is comparable to the security level provided
by the randomized procedure. However, as we allow the number of samples to
increase, the randomized procedure provides a higher probabilistic security
level.Comment: A short version of this paper (without proofs) will be presented at
the 18th IFAC World Congress (IFAC 2011), Milan (Italy), August 28-September
2, 201
Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit
A comprehensive investigation of the frequency-noise spectral density of a
free-running mid-infrared quantum-cascade laser is presented for the first
time. It provides direct evidence of the leveling of this noise down to a white
noise plateau, corresponding to an intrinsic linewidth of a few hundred Hz. The
experiment is in agreement with the most recent theory on the fundamental
mechanism of line broadening in quantum-cascade lasers, which provides a new
insight into the Schawlow-Townes formula and predicts a narrowing beyond the
limit set by the radiative lifetime of the upper level.Comment: 4 pages, 4 figure
Massive creation of entangled exciton states in semiconductor quantum dots
An intense laser pulse propagating in a medium of inhomogeneously broadened
quantum dots massively creates entangled exciton states. After passage of the
pulse all single-exciton states remain unpopulated (self-induced transparency)
whereas biexciton coherence (exciton entanglement) is generated through
two-photon transitions. We propose several experimental techniques for the
observation of such unexpected behavior
Radiative corrections to the excitonic molecule state in GaAs microcavities
The optical properties of excitonic molecules (XXs) in GaAs-based quantum
well microcavities (MCs) are studied, both theoretically and experimentally. We
show that the radiative corrections to the XX state, the Lamb shift
and radiative width , are
large, about of the molecule binding energy , and
definitely cannot be neglected. The optics of excitonic molecules is dominated
by the in-plane resonant dissociation of the molecules into outgoing
1-mode and 0-mode cavity polaritons. The later decay channel,
``excitonic molecule 0-mode polariton + 0-mode
polariton'', deals with the short-wavelength MC polaritons invisible in
standard optical experiments, i.e., refers to ``hidden'' optics of
microcavities. By using transient four-wave mixing and pump-probe
spectroscopies, we infer that the radiative width, associated with excitonic
molecules of the binding energy meV, is
meV in the microcavities and
meV in a reference GaAs single quantum
well (QW). We show that for our high-quality quasi-two-dimensional
nanostructures the limit, relevant to the XX states, holds at
temperatures below 10 K, and that the bipolariton model of excitonic molecules
explains quantitatively and self-consistently the measured XX radiative widths.
We also find and characterize two critical points in the dependence of the
radiative corrections against the microcavity detuning, and propose to use the
critical points for high-precision measurements of the molecule bindingenergy
and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.
Indistinguishability of independent single photons
The indistinguishability of independent single photons is presented by
decomposing the single photon pulse into the mixed state of different transform
limited pulses. The entanglement between single photons and outer environment
or other photons induces the distribution of the center frequencies of those
transform limited pulses and makes photons distinguishable. Only the single
photons with the same transform limited form are indistinguishable. In details,
the indistinguishability of single photons from the solid-state quantum emitter
and spontaneous parametric down conversion is examined with two-photon
Hong-Ou-Mandel interferometer. Moreover, experimental methods to enhance the
indistinguishability are discussed, where the usage of spectral filter is
highlighted.Comment: 6 pages, 3 figure
Tihonov theory and center manifolds for inhibitory mechanisms in enzyme kinetics
Abstract In this paper we study the chemical reaction of inhibition, determine the appropriate parameter ε for the application of Tihonov's Theorem, compute explicitly the equations of the center manifold of the system and find sufficient conditions to guarantee that in the phase space the curves which relate the behavior of the complexes to the substrates by means of the tQSSA are asymptotically equivalent to the center manifold of the system. Some numerical results are discussed
Resonant nature of phonon-induced damping of Rabi oscillations in quantum dots
Optically controlled coherent dynamics of charge (excitonic) degrees of
freedom in a semiconductor quantum dot under the influence of lattice dynamics
(phonons) is discussed theoretically. We show that the dynamics of the lattice
response in the strongly non-linear regime is governed by a semiclassical
resonance between the phonon modes and the optically driven dynamics. We stress
on the importance of the stability of intermediate states for the truly
coherent control.Comment: 4 pages, 2 figures; final version; moderate changes, new titl
Chemically-specific dual/differential CARS micro-spectroscopy of saturated and unsaturated lipid droplets
We have investigated the ability of dual-frequency Coherent Antistokes Raman Scattering (D-CARS) micro-spectroscopy, based on femtosecond pulses (100 fs or 5 fs) spectrally focussed by glass dispersion, to distinguish the chemical composition of micron-sized lipid droplets consisting of different triglycerides types (poly-unsaturated glyceryl trilinolenate, mono-unsaturated glyceryl trioleate and saturated glyceryl tricaprylate and glyceryl tristearate) in a rapid and label-free way. A systematic comparison of Raman spectra with CARS and D-CARS spectra was used to identify D-CARS spectral signatures which distinguish the disordered poly-unsaturated lipids from the more ordered saturated ones both in the CH-stretch vibration region and in the fingerprint region, without the need for lengthy CARS multiplex acquisition and analysis. D-CARS images of the lipid droplets at few selected wavenumbers clearly resolved the lipid composition differences, and exemplify the potential of this technique for label-free chemically selective rapid imaging of cytosolic lipid droplets in living cell
Exciton formation and relaxation in GaAs epilayers
Exciton formation and relaxation in GaAs bulk epilayers have been studied by means of time-resolved photoluminescence techniques. It is found that the time evolution of the free exciton luminescence, nonresonantly excited at low temperature and low intensity, is extremely slow, with a rise time of the order of 1 ns and a decay time of several ns. Simulations based on Monte Carlo solution of the set of coupled Boltzmann-like equations for free carriers and excitons show a nice agreement with the experimental data, and suggest a dominant role played by acoustic phonons in the exciton relaxation
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