860 research outputs found
Spontaneous decay of an excited atom placed near a rectangular plate
Using the Born expansion of the Green tensor, we consider the spontaneous
decay rate of an excited atom placed in the vicinity of a rectangular plate. We
discuss the limitations of the commonly used simplifying assumption that the
plate extends to infinity in the lateral directions and examine the effects of
the atomic dipole moment orientation, atomic position, and plate boundary and
thickness on the atomic decay rate. In particular, it is shown that in the
boundary region, the spontaneous decay rate can be strongly modified.Comment: 5 pages, 5 figure
Resonant Energy Exchange between Atoms in Dispersing and Absorbing Surroundings
Within the framework of quantization of the macroscopic electromagnetic
field, a master equation describing both the resonant dipole-dipole interaction
(RDDI) and the resonant atom-field interaction (RAFI) in the presence of
dispersing and absorbing macroscopic bodies is derived, with the relevant
couplings being expressed in terms of the surroundings-assisted Green tensor.
It is shown that under certain conditions the RDDI can be regarded as being
governed by an effective Hamiltonian. The theory, which applies to both weak
and strong atom-field coupling, is used to study the resonant energy exchange
between two (two-level) atoms sharing initially a single excitation. In
particular, it is shown that in the regime of weak atom-field coupling there is
a time window, where the energy transfer follows a transfer-rate law of the
type obtained by ordinary second-order perturbation theory. Finally, the
spectrum of the light emitted during the energy transfer is studied and the
line splittings are discussed.Comment: 9 pages, 5 figs, Proceedings of ICQO'2002, Raubichi, to appear in
Optics and Spectroscop
Oscillator model for dissipative QED in an inhomogeneous dielectric
The Ullersma model for the damped harmonic oscillator is coupled to the
quantised electromagnetic field. All material parameters and interaction
strengths are allowed to depend on position. The ensuing Hamiltonian is
expressed in terms of canonical fields, and diagonalised by performing a
normal-mode expansion. The commutation relations of the diagonalising operators
are in agreement with the canonical commutation relations. For the proof we
replace all sums of normal modes by complex integrals with the help of the
residue theorem. The same technique helps us to explicitly calculate the
quantum evolution of all canonical and electromagnetic fields. We identify the
dielectric constant and the Green function of the wave equation for the
electric field. Both functions are meromorphic in the complex frequency plane.
The solution of the extended Ullersma model is in keeping with well-known
phenomenological rules for setting up quantum electrodynamics in an absorptive
and spatially inhomogeneous dielectric. To establish this fundamental
justification, we subject the reservoir of independent harmonic oscillators to
a continuum limit. The resonant frequencies of the reservoir are smeared out
over the real axis. Consequently, the poles of both the dielectric constant and
the Green function unite to form a branch cut. Performing an analytic
continuation beyond this branch cut, we find that the long-time behaviour of
the quantised electric field is completely determined by the sources of the
reservoir. Through a Riemann-Lebesgue argument we demonstrate that the field
itself tends to zero, whereas its quantum fluctuations stay alive. We argue
that the last feature may have important consequences for application of
entanglement and related processes in quantum devices.Comment: 24 pages, 1 figur
Factored state-abstract hidden Markov models for activity recognition using pervasive multi-modal sensors
Current probabilistic models for activity recognition do not incorporate much sensory input data due to the problem of state space explosion. In this paper, we propose a model for activity recognition, called the Factored State-Abtract Hidden Markov Model (FS-AHMM) to allow us to integrate many sensors for improving recognition performance. The proposed FS-AHMM is an extension of the Abstract Hidden Markov Model which applies the concept of factored state representations to compactly represent the state transitions. The parameters of the FS-AHMM are estimated using the EM algorithm from the data acquired through multiple multi-modal sensors and cameras. The model is evaluated and compared with other existing models on real-world data. The results show that the proposed model outperforms other models and that the integrated sensor information helps in recognizing activity more accurately
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