295 research outputs found
Temporal and diffraction effects in entanglement creation in an optical cavity
A practical scheme for entanglement creation between distant atoms located
inside a single-mode optical cavity is discussed. We show that the degree of
entanglement and the time it takes for the entanglement to reach its optimum
value is a sensitive function the initial conditions and the position of the
atoms inside the cavity mode. It is found that the entangled properties of the
two atoms can readily be extracted from dynamics of a simple two-level system.
Effectively, we engineer two coupled qubits whose the dynamics are analogous to
that of a driven single two-level system. It is found that spatial variations
of the coupling constants actually help to create transient entanglement which
may appear on the time scale much longer than that predicted for the case of
equal coupling constants. When the atoms are initially prepared in an entangled
state, they may remain entangled for all times. We also find that the
entanglement exhibits an interesting phenomenon of diffraction when the the
atoms are located between the nodes and antinodes of the cavity mode. The
diffraction pattern of the entanglement varies with time and we explain this
effect in terms of the quantum property of complementarity, which is manifested
as a tradeoff between the knowledge of energy of the exchanged photon versus
the evolution time of the system.Comment: Phys. Rev. A75, 042307 (2007
Effect of retardation on the dynamics of entanglement between atoms
The role of retardation in the entanglement dynamics of two distant atoms
interacting with a multi-mode field of a ring cavity is discussed. The
retardation is associated with a finite time required for light to travel
between the atoms located at a finite distance and between the atoms and the
cavity boundaries. We explore features in the concurrence indicative of
retardation and show how these features evolve depending on the initial state
of the system, distance between the atoms and the number of modes to which the
atoms are coupled. In particular, we consider the short-time and the long time
dynamics for both the multi- and sub-wavelength distances between the atoms. It
is found that the retardation effects can qualitatively modify the entanglement
dynamics of the atoms not only at multi- but also at sub-wavelength distances.
We follow the temporal evolution of the concurrence and find that at short
times of the evolution the retardation induces periodic sudden changes of
entanglement. To analyze where the entanglement lies in the space spanned by
the state vectors of the system, we introduce the collective Dicke states of
the atomic system that explicitly account for the sudden changes as a periodic
excitation of the atomic system to the maximally entangled symmetric state. At
long times, the retardation gives rise to periodic beats in the concurrence
that resemble the phenomenon of collapses and revivals in the Jaynes-Cummings
model. In addition, we identify parameter values and initial conditions at
which the atoms remain separable or are entangled without retardation during
the entire evolution time, but exhibit the phenomena of sudden birth and sudden
death of entanglement when the retardation is included.Comment: 16 pages, 14 figure
Spin squeezing as a measure of entanglement in a two qubit system
We show that two definitions of spin squeezing extensively used in the
literature [M. Kitagawa and M. Ueda, Phys. Rev. A {\bf 47}, 5138 (1993) and
D.J. Wineland {\it et al.}, Phys. Rev. A {\bf 50}, 67 (1994)] give different
predictions of entanglement in the two-atom Dicke system. We analyze
differences between the definitions and show that the Kitagawa and Ueda's spin
squeezing parameter is a better measure of entanglement than the commonly used
spectroscopic spin squeezing parameter. We illustrate this relation by
examining different examples of a driven two-atom Dicke system in which spin
squeezing and entanglement arise dynamically. We give an explanation of the
source of the difference in the prediction of entanglement using the negativity
criterion for entanglement. For the examples discussed, we find that the
Kitagawa and Ueda's spin squeezing parameter is the sufficient and necessary
condition for entanglement.Comment: 5 pages, 4 figure
Entanglement and nonlocality versus spontaneous emission in two-atom system
We study evolution of entanglement of two two-level atoms in the presence of
dissipation caused by spontaneous emission. We find explicit fromulas for the
amount of entanglement as a function of time, in the case of destruction of the
initial entanglement and possible creation of a transient entanglement between
atoms. We also discuss how spontaneous emission influences nonlocality of
states expressed by violation of Bell - CHSH inequality. It is shown that
evolving system very quickly becomes local, even if entanglement is still
present or produced.Comment: 15 pages, 7 figure
Performing Conquest and Resistance in the Streets of Eighteenth Century PotosĂ: Identity and Artifice in the Cityscapes of Gaspar Miguel de BerrĂo and Melchor PĂ©rez de HolguĂn
This thesis examines the ways in which PotosĂ\u27s two most influential colonial artists represented the urban dynamics of race, class and labor in their depictions of the Andean \u27City of Silver\u27 during the eighteenth century, when silver production, profits and population were dramatically declining
Collective coherent population trapping in a thermal field
We analyzed the efficiency of coherent population trapping (CPT) in a
superposition of the ground states of three-level atoms under the influence of
the decoherence process induced by a broadband thermal field. We showed that in
a single atom there is no perfect CPT when the atomic transitions are affected
by the thermal field. The perfect CPT may occur when only one of the two atomic
transitions is affected by the thermal field. In the case when both atomic
transitions are affected by the thermal field, we demonstrated that regardless
of the intensity of the thermal field the destructive effect on the CPT can be
circumvented by the collective behavior of the atoms. An analytic expression
was obtained for the populations of the upper atomic levels which can be
considered as a measure of the level of thermal decoherence. The results show
that the collective interaction between the atoms can significantly enhance the
population trapping in that the population of the upper state decreases with
increased number of atoms. The physical origin of this feature was explained by
the semiclassical dressed atom model of the system. We introduced the concept
of multiatom collective coherent population trapping by demonstrating the
existence of collective (entangled) states whose storage capacity is larger
than that of the equivalent states of independent atoms.Comment: Accepted for publication in Phys. Rev.
Stationary two-atom entanglement induced by nonclassical two-photon correlations
A system of two two-level atoms interacting with a squeezed vacuum field can
exhibit stationary entanglement associated with nonclassical two-photon
correlations characteristic of the squeezed vacuum field. The amount of
entanglement present in the system is quantified by the well known measure of
entanglement called concurrence. We find analytical formulas describing the
concurrence for two identical and nonidentical atoms and show that it is
possible to obtain a large degree of steady-state entanglement in the system.
Necessary conditions for the entanglement are nonclassical two-photon
correlations and nonzero collective decay. It is shown that nonidentical atoms
are a better source of stationary entanglement than identical atoms. We discuss
the optimal physical conditions for creating entanglement in the system, in
particular, it is shown that there is an optimal and rather small value of the
mean photon number required for creating entanglement.Comment: 17 pages, 5 figure
Deterministic creation of stationary entangled states by dissipation
We propose a practical physical system for creation of a stationary
entanglement by dissipation without employing the environment engineering
techniques. The system proposed is composed of two perfectly distinguishable
atoms, through their significantly different transition frequencies, with only
one atom addressed by an external laser field. We show that the arrangement
would easily be realized in practice by trapping the atoms at the distance
equal to the quarter-wavelength of a standing-wave laser field and locating one
of the atoms at a node and the other at the successive antinode of the wave.
The undesirable dipole-dipole interaction between the atoms, that could be
large at this small distance, is adjusted to zero by a specific initial
preparation of the atoms or by a specific polarization of the atomic dipole
moments. Following this arrangement, we show that the dissipative relaxation
can create a stationary entanglement on demand by tuning the Rabi frequency of
the laser field to the difference between the atomic transition frequencies.
The laser field dresses the atom and we identify that the entangled state
occurs when the frequency of one of the Rabi sidebands of the driven atom tunes
to frequency of the undriven atom. It is also found that this system behaves as
a cascade open system where the fluorescence from the dressed atom drives the
other atom with no feedback.Comment: Published versio
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
Response of a two-level atom to a narrow-bandwidth squeezed-vacuum excitation
Using the coupled-system approach we calculate the optical spectra of the fluorescence and transmitted fields of a two-level atom driven by a squeezed vacuum of bandwidths smaller than the natural atomic linewidth. We find that in this regime of squeezing bandwidths the spectra exhibit unique features, such as a hole burning and a three-peak structure, which do not appear for a broadband excitation. We show that the features are unique to the quantum nature of the driving squeezed vacuum field and donor appear when the atom is driven by a classically squeezed field. We find that a quantum squeezed-vacuum field produces squeezing in the emitted fluorescence field which appears only in the squeezing spectrum while there is no squeezing in the total field. We also discuss a nonresonant excitation and find that depending on the squeezing bandwidth there is a peak or a hole in the spectrum at a frequency corresponding to a three-wave-mixing process. The hole appears only for a broadband excitation and results from the strong correlations between squeezed-vacuum photons
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