4,667 research outputs found
Single photon absorption and dynamic control of a coupled quantum dot-cavity system
We theoretically investigate the dynamic interaction of a quantum dot in a
nanocavity with timesymmetric single photon pulses. The simulations, based on a
wavefunction approach, reveal that almost perfect single photon absorption
occurs for quantum dot-cavity systems operating on the edge between strong and
weak coupling regime. The computed maximum absorptions probability is close to
unity for pulses with a typical length comparable to the half of the Rabi
period. Furthermore, the dynamic control of the quantum dot energy via electric
fields allows the freezing of the light-matter interaction leaving the quantum
dot in its excited state. Shaping of single photon wavepackets by the electric
field control is limited by the occurrence of chirping of the single photon
pulse. This understanding of the interaction of single photon pulses with the
quantum dot-cavity system provides the basis for the development of advanced
protocols for quantum information processing in the solid state.Comment: 7 pages, 4 figure
Relationships Between the Performance of Time/Frequency Standards and Navigation/Communication Systems
The relationship between system performance and clock or oscillator performance is discussed. Tradeoffs discussed include: short term stability versus bandwidth requirements; frequency accuracy versus signal acquisition time; flicker of frequency and drift versus resynchronization time; frequency precision versus communications traffic volume; spectral purity versus bit error rate, and frequency standard stability versus frequency selection and adjustability. The benefits and tradeoffs of using precise frequency and time signals are various levels of precision and accuracy are emphasized
Low-energy electron scattering by tetrahydrofuran
Cross sections for elastic scattering of low-energy electrons by tetrahydrofuran, a prototype for the furanose ring found in the backbone of DNA, have been measured and calculated over a wide energy range, with an emphasis on energies below 6 eV, where previous data are scarce. The measurements employ a thin-aperture version of the relative-flow method, while the calculations employ the Schwinger multichannel method with an extensive treatment of polarization effects. Comparisons with earlier results, both experimental and theoretical, are presented and discussed. A proper accounting for the strong permanent electric dipole of tetrahydrofuran is found to be essential to obtaining reliable cross sections, especially at energies below 5 eV
Quantum turbulence in condensate collisions: an application of the classical field method
We apply the classical field method to simulate the production of correlated
atoms during the collision of two Bose-Einstein condensates. Our
non-perturbative method includes the effect of quantum noise, and provides for
the first time a theoretical description of collisions of high density
condensates with very large out-scattered fractions. Quantum correlation
functions for the scattered atoms are calculated from a single simulation, and
show that the correlation between pairs of atoms of opposite momentum is rather
small. We also predict the existence of quantum turbulence in the field of the
scattered atoms--a property which should be straightforwardly measurable.Comment: 5 pages, 3 figures: Rewritten text, replaced figure
Number-Phase Wigner Representation for Efficient Stochastic Simulations
Phase-space representations based on coherent states (P, Q, Wigner) have been
successful in the creation of stochastic differential equations (SDEs) for the
efficient stochastic simulation of high dimensional quantum systems. However
many problems using these techniques remain intractable over long integrations
times. We present a number-phase Wigner representation that can be unraveled
into SDEs. We demonstrate convergence to the correct solution for an anharmonic
oscillator with small dampening for significantly longer than other phase space
representations. This process requires an effective sampling of a non-classical
probability distribution. We describe and demonstrate a method of achieving
this sampling using stochastic weights.Comment: 7 pages, 1 figur
Quantum Trajectory method for the Quantum Zeno and anti-Zeno effects
We perform stochastic simulations of the quantum Zeno and anti-Zeno effects
for two level system and for the decaying one. Instead of simple projection
postulate approach, a more realistic model of a detector interacting with the
environment is used. The influence of the environment is taken into account
using the quantum trajectory method. The simulation of the measurement for a
single system exhibits the probabilistic behavior showing the collapse of the
wave-packet. When a large ensemble is analysed using the quantum trajectory
method, the results are the same as those produced using the density matrix
method. The results of numerical calculations are compared with the analytical
expressions for the decay rate of the measured system and a good agreement is
found. Since the analytical expressions depend on the duration of the
measurement only, the agreement with the numerical calculations shows that
otherparameters of the model are not important.Comment: 12 figures, accepted for publication in Phys. Rev. A replaced with
single-spaced versio
Generalized Limits for Parameter Sensitivity via Quantum Ziv-Zakai Bound
We study the generalized limit for parameter sensitivity in quantum
estimation theory considering the effects of repeated and adaptive
measurements. Based on the quantum Ziv-Zakai bound, we derive some lower bounds
for parameter sensitivity when the Hamiltonian of system is unbounded and when
the adaptive measurements are implemented on the system. We also prove that the
parameter sensitivity is bounded by the limit of the minimum detectable
parameter. In particular, we examine several known states in quantum phase
estimation with non-interacting photons, and show that they can not perform
better than Heisenberg limit in a much simpler way with our result.Comment: 8pages, 5 figure
Quantum effects on the dynamics of a two-mode atom-molecule Bose-Einstein condensate
We study the system of coupled atomic and molecular condensates within the
two-mode model and beyond mean-field theory (MFT). Large amplitude
atom-molecule coherent oscillations are shown to be damped by the rapid growth
of fluctuations near the dynamically unstable molecular mode. This result
contradicts earlier predictions about the recovery of atom-molecule
oscillations in the two-mode limit. The frequency of the damped oscillation is
also shown to scale as with the total number of atoms ,
rather than the expected pure scaling. Using a linearized model, we
obtain analytical expressions for the initial depletion of the molecular
condensate in the vicinity of the instability, and show that the important
effect neglected by mean field theory is an initially non-exponential
`spontaneous' dissociation into the atomic vacuum. Starting with a small
population in the atomic mode, the initial dissociation rate is sensitive to
the exact atomic amplitudes, with the fastest (super-exponential) rate observed
for the entangled state, formed by spontaneous dissociation.Comment: LaTeX, 5 pages, 3 PostScript figures, uses REVTeX and epsfig,
submitted to Physical Review A, Rapid Communication
Effect of an atom on a quantum guided field in a weakly driven fiber-Bragg-grating cavity
We study the interaction of an atom with a quantum guided field in a weakly
driven fiber-Bragg-grating (FBG) cavity. We present an effective Hamiltonian
and derive the density-matrix equations for the combined atom-cavity system. We
calculate the mean photon number, the second-order photon correlation function,
and the atomic excited-state population. We show that, due to the confinement
of the guided cavity field in the fiber cross-section plane and in the space
between the FBG mirrors, the presence of the atom in the FBG cavity can
significantly affect the mean photon number and the photon statistics even
though the cavity finesse is moderate, the cavity is long, and the probe field
is weak.Comment: Accepted for Phys. Rev.
- …