5,159 research outputs found
Generation and purification of maximally-entangled atomic states in optical cavities
We present a probabilistic scheme for generating and purifying
maximally-entangled states of two atoms inside an optical cavity via no-photon
detection in the output cavity mode, where ideal detectors may not be required.
The intermediate mixed states can be continuously "filtered" so as to violate
Bell inequalities in a parametrized manner. The scheme relies on an additional
strong-driving field that yields unusual dynamics in cavity QED experiments,
simultaneously realizing Jaynes-Cummings and anti-Jaynes-Cummings interactions.Comment: 4 pages and 3 figure
Instantaneous Measurement of field quadrature moments and entanglement
We present a method of measuring expectation values of quadrature moments of
a multimode field through two-level probe ``homodyning''. Our approach is based
on an integral transform formalism of measurable probe observables, where
analytically derived kernels unravel efficiently the required field information
at zero interaction time, minimizing decoherence effects. The proposed scheme
is suitable for fields that, while inaccessible to a direct measurement, enjoy
one and two-photon Jaynes-Cummings interactions with a two-level probe, like
spin, phonon, or cavity fields. Available data from previous experiments are
used to confirm our predictions.Comment: 4 pages, no figures, modified version with experimental estimation
Strong-driving-assisted multipartite entanglement in cavity QED
We propose a method of generating multipartite entanglement by considering
the interaction of a system of N two-level atoms in a cavity of high quality
factor with a strong classical driving field. It is shown that, with a
judicious choice of the cavity detuning and the applied coherent field
detuning, vacuum Rabi coupling produces a large number of important
multipartite entangled states. It is even possible to produce entangled states
involving different cavity modes. Tuning of parameters also permits us to
switch from Jaynes-Cummings to anti-Jaynes-Cummings like interaction.Comment: Last version with minor changes and added references. Accepted for
publication in Phys. Rev. Letter
Measure of phonon-number moments and motional quadratures through infinitesimal-time probing of trapped ions
A method for gaining information about the phonon-number moments and the
generalized nonlinear and linear quadratures in the motion of trapped ions (in
particular, position and momentum) is proposed, valid inside and outside the
Lamb-Dicke regime. It is based on the measurement of first time derivatives of
electronic populations, evaluated at the motion-probe interaction time t=0. In
contrast to other state-reconstruction proposals, based on measuring Rabi
oscillations or dispersive interactions, the present scheme can be performed
resonantly at infinitesimal short motion-probe interaction times, remaining
thus insensitive to decoherence processes.Comment: 10 pages. Accepted in JPhys
Sequential Generation of Matrix-Product States in Cavity QED
We study the sequential generation of entangled photonic and atomic
multi-qubit states in the realm of cavity QED. We extend the work of C. Schoen
et al. [Phys. Rev. Lett. 95, 110503 (2005)], where it was shown that all states
generated in a sequential manner can be classified efficiently in terms of
matrix-product states. In particular, we consider two scenarios: photonic
multi-qubit states sequentially generated at the cavity output of a
single-photon source and atomic multi-qubit states generated by their
sequential interaction with the same cavity mode.Comment: 11 page
Numerical time propagation of quantum systems in radiation fields
Atoms, molecules or excitonic quasiparticles, for which excitations are
induced by external radiation fields and energy is dissipated through radiative
decay, are examples of driven open quantum systems. We explain the use of
commutator-free exponential time-propagators for the numerical solution of the
associated Schr\"odinger or master equations with a time-dependent Hamilton
operator. These time-propagators are based on the Magnus series but avoid the
computation of commutators, which makes them suitable for the efficient
propagation of systems with a large number of degrees of freedom. We present an
optimized fourth order propagator and demonstrate its efficiency in comparison
to the direct Runge-Kutta computation. As an illustrative example we consider
the parametrically driven dissipative Dicke model, for which we calculate the
periodic steady state and the optical emission spectrum.Comment: 23 pages, 11 figure
Macroscopic Interference Effects in Resonant Cavities
We investigate the possibility of interference effects induced by macroscopic
quantum-mechanical superpositions of almost othogonal coherent states - a
Schroedinger cats state - in a resonant microcavity. Despite the fact that a
single atom, used as a probe of the cat state, on the average only change the
mean number of photons by one unit, we show that this single atom can change
the system drastically. Interference between the initial and almost orthogonal
macroscopic quantum states of the radiation field can now take place.
Dissipation under current experimental conditions is taken into account and it
is found that this does not necessarily change the intereference effects
dramatically.Comment: 20 pages, 3 figure
Cavity-assisted spontaneous emission as a single-photon source: Pulse shape and efficiency of one-photon Fock state preparation
Within the framework of exact quantum electrodynamics in dispersing and
absorbing media, we have studied the quantum state of the radiation emitted
from an initially in the upper state prepared two-level atom in a high-
cavity, including the regime where the emitted photon belongs to a wave packet
that simultaneously covers the areas inside and outside the cavity. For both
continuing atom--field interaction and short-term atom--field interaction, we
have determined the spatio-temporal shape of the excited outgoing wave packet
and calculated the efficiency of the wave packet to carry a one-photon Fock
state. Furthermore, we have made contact with quantum noise theories where the
intracavity field and the field outside the cavity are regarded as
approximately representing independent degrees of freedom such that two
separate Hilbert spaces can be introduced.Comment: 16 pages, 7 eps figures; improved version as submitted to Phys. Rev.
The Early Bird Catches The Term: Combining Twitter and News Data For Event Detection and Situational Awareness
Twitter updates now represent an enormous stream of information originating
from a wide variety of formal and informal sources, much of which is relevant
to real-world events. In this paper we adapt existing bio-surveillance
algorithms to detect localised spikes in Twitter activity corresponding to real
events with a high level of confidence. We then develop a methodology to
automatically summarise these events, both by providing the tweets which fully
describe the event and by linking to highly relevant news articles. We apply
our methods to outbreaks of illness and events strongly affecting sentiment. In
both case studies we are able to detect events verifiable by third party
sources and produce high quality summaries
Monitoring the Dipole-Dipole Interaction via Quantum Jumps of Individual Atoms
The emission characteristics in the fluorescence of two laser-driven dipole-dipole-interacting three level atoms is investigated. When the light from both atoms is detected separately a correlation of the emission processes is observed in dependence of the dipole-dipole interaction. This opens the possibility to investigate the dipole-dipole interaction through the emission behavior. We present Monte-Carlo simulations which are in good agreement with the analytic solutions
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