203 research outputs found
Entanglement created by spontaneously generated coherence
We propose a scheme able to generate on demand a steady-state entanglement
between two non-degenerate cavity modes. The scheme relies on the interaction
of the cavity modes with driven two or three-level atoms which act as a coupler
to build entanglement between the modes. We show that in the limit of a strong
driving, crucial for the generation of entanglement between the modes is to
imbalance populations of the dressed states of the driven atomic transition. In
the case of a three-level V-type atom, we find that a stationary entanglement
can be created on demand by tuning the Rabi frequency of the driving field to
the difference between the atomic transition frequencies. The resulting
degeneracy of the energy levels together with the spontaneously generated
coherence generates a steady-state entanglement between the cavity modes. It is
shown that the condition for the maximal entanglement coincides with the
collapse of the atomic system into a pure trapping state. We also show that the
creation of entanglement depends strongly on the mutual polarization of the
transition atomic dipole moments.Comment: Published versio
Three-level atom in a broadband squeezed vacuum field. II. Applications
Using the formalism developed earlier, we treat spontaneous emission from a three-level atom (ladder system) interacting with a broadband squeezed vacuum field. We obtain expressions for the transient and steady-state populations of the atomic levels with the conditions that the atom interacts with either a multimode perfect squeezed vacuum field, or a three-dimensional vacuum field in which the squeezed modes lie within a solid angle over which squeezing is propagated. The results are compared with those obtained for the atom interacting with a thermal field. We show that in the perfect case the first excited state is not populated when the squeezed vacuum field is in a minimum-uncertainty squeezed state. Moreover, the second excited state can have a steady-state population larger than 1/2. These features are completely absent when the atom interacts with the thermal field. In addition, for a low-intensity squeezed vacuum field the population in the second excited state exhibits a linear rather than quadratic dependence on the intensity of the squeezed vacuum field. In the three-dimensional case the presence of unsqueezed modes considerably reduces the effect of squeezing on spontaneous emission. However, a significant reduction in a population of the first excited state and a population larger than 1/2 in the second excited state can be achieved provided the squeezing is propagated over a large solid angle. We also discuss the effect of the two-photon detuning between the double carrier frequency of the squeezing and atomic transition frequency 3 on the steady-state atomic population
Comment on Phase-sensitive population decay: the two-atom Dicke model in a broadband squeezed vacuum
A recent paper [G. M. Palma and P. L. Knight, Phys. Rev. A 39, 1962 (1989)] has stated that the two-atom Dicke model exhibits phase-dependent population decay. In this Comment we show that population decay is invariant with respect to the input phase angle of squeezing in the two-atom Dicke model
Dipole-dipole interaction between orthogonal dipole moments in time-dependent geometries
In two nearby atoms, the dipole-dipole interaction can couple transitions
with orthogonal dipole moments. This orthogonal coupling accounts for a number
of interesting effects, but strongly depends on the geometry of the setup.
Here, we discuss several setups of interest where the geometry is not fixed,
such as particles in a trap or gases, by averaging over different sets of
geometries. Two averaging methods are compared. In the first method, it is
assumed that the internal electronic evolution is much faster than the change
of geometry, whereas in the second, it is vice versa. We find that the
orthogonal coupling typically survives even extensive averaging over different
geometries, albeit with qualitatively different results for the two averaging
methods. Typically, one- and two-dimensional averaging ranges modelling, e.g.,
low-dimensional gases, turn out to be the most promising model systems.Comment: 11 pages, 14 figure
Quantum interference in optical fields and atomic radiation
We discuss the connection between quantum interference effects in optical
beams and radiation fields emitted from atomic systems. We illustrate this
connection by a study of the first- and second-order correlation functions of
optical fields and atomic dipole moments. We explore the role of correlations
between the emitting systems and present examples of practical methods to
implement two systems with non-orthogonal dipole moments. We also derive
general conditions for quantum interference in a two-atom system and for a
control of spontaneous emission. The relation between population trapping and
dark states is also discussed. Moreover, we present quantum dressed-atom models
of cancellation of spontaneous emission, amplification on dark transitions,
fluorescence quenching and coherent population trapping.Comment: To be published in Journal of Modern Optics Special Issue on Quantum
Interferenc
First-order coherence versus entanglement in a nano-mechanical cavity
The coherence and correlation properties of effective bosonic modes of a
nano-mechanical cavity composed of an oscillating mirror and containing an
optical lattice of regularly trapped atoms are studied. The system is modeled
as a three-mode system, two orthogonal polariton modes representing the coupled
optical lattice and the cavity mode, and one mechanical mode representing the
oscillating mirror. We examine separately the cases of two-mode and three-mode
interactions which are distinguished by a suitable tuning of the mechanical
mode to the polariton mode frequencies. In the two-mode case, we find that the
occurrence of entanglement between one of the polariton modes and the
mechanical mode is highly sensitive to the presence of the first-order
coherence between the modes. In particular, the creation of the first-order
coherence among the modes is achieved at the expense of entanglement between
the modes. In the three-mode case, we show that no entanglement is created
between the independent polariton modes if both modes are coupled to the
mechanical mode by the parametric interaction. There is no entanglement between
the polaritons even if the oscillating mirror is damped by a squeezed vacuum
field. The interaction creates the first-order coherence between the polaritons
and the degree of coherence can, in principle, be as large as unity. This
demonstrates that the oscillating mirror can establish the first-order
coherence between two independent thermal modes. A further analysis shows that
two independent thermal modes can be made entangled in the system only when one
of the modes is coupled to the intermediate mode by a parametric interaction
and the other is coupled by a linear-mixing interaction.Comment: Published versio
Entanglement and spin squeezing in the two-atom Dicke model
We analyze the relation between the entanglement and spin-squeezing parameter
in the two-atom Dicke model and identify the source of the discrepancy recently
reported by Banerjee and Zhou et al that one can observe entanglement without
spin squeezing. Our calculations demonstrate that there are two criteria for
entanglement, one associated with the two-photon coherences that create
two-photon entangled states, and the other associated with populations of the
collective states. We find that the spin-squeezing parameter correctly predicts
entanglement in the two-atom Dicke system only if it is associated with
two-photon entangled states, but fails to predict entanglement when it is
associated with the entangled symmetric state. This explicitly identifies the
source of the discrepancy and explains why the system can be entangled without
spin-squeezing. We illustrate these findings in three examples of the
interaction of the system with thermal, classical squeezed vacuum and quantum
squeezed vacuum fields.Comment: 7 pages, 1 figur
Initial-Phase Spectroscopy as a Control of Entangled Systems
We introduce the concept of initial-phase spectroscopy as a control of the
dynamics of entangled states encoded into a two-atom system interacting with a
broadband squeezed vacuum field. We illustrate our considerations by examining
the transient spectrum of the field emitted by two systems, the small sample
(Dicke) and the spatially extended (non-Dicke) models. It is found that the
shape of the spectral components depends crucially on the relative phase
between the initial entangled state and the squeezed field. We follow the
temporal evolution of the spectrum and show that depending on the relative
phase a hole burning can occur in one of the two spectral lines. We compare the
transient behavior of the spectrum with the time evolution of the initial
entanglement and find that the hole burning can be interpreted as a
manifestation of the phenomenon of entanglement sudden death. In addition, we
find that in the case of the non-Dicke model, the collective damping rate may
act like an artificial tweezer that rotates the phase of the squeezed field.Comment: 20 pages, 9 figure
Theory of quantum fluctuations of optical dissipative structures and its application to the squeezing properties of bright cavity solitons
We present a method for the study of quantum fluctuations of dissipative
structures forming in nonlinear optical cavities, which we illustrate in the
case of a degenerate, type I optical parametric oscillator. The method consists
in (i) taking into account explicitly, through a collective variable
description, the drift of the dissipative structure caused by the quantum
noise, and (ii) expanding the remaining -internal- fluctuations in the
biorthonormal basis associated to the linear operator governing the evolution
of fluctuations in the linearized Langevin equations. We obtain general
expressions for the squeezing and intensity fluctuations spectra. Then we
theoretically study the squeezing properties of a special dissipative
structure, namely, the bright cavity soliton. After reviewing our previous
result that in the linear approximation there is a perfectly squeezed mode
irrespectively of the values of the system parameters, we consider squeezing at
the bifurcation points, and the squeezing detection with a plane--wave local
oscillator field, taking also into account the effect of the detector size on
the level of detectable squeezing.Comment: 10 figure
- …