264 research outputs found
Dark periods and revivals of entanglement in a two qubit system
In a recent paper Yu and Eberly [Phys. Rev. Lett. {\bf 93}, 140404 (2004)]
have shown that two initially entangled and afterwards not interacting qubits
can become completely disentangled in a finite time. We study transient
entanglement between two qubits coupled collectively to a multimode vacuum
field and find an unusual feature that the irreversible spontaneous decay can
lead to a revival of the entanglement that has already been destroyed. The
results show that this feature is independent of the coherent dipole-dipole
interaction between the atoms but it depends critically on whether or not the
collective damping is present. We show that the ability of the system to
revival entanglement via spontaneous emission relies on the presence of very
different timescales for the evolution of the populations of the collective
states and coherence between them.Comment: 4 pages, 3 figure
Delayed (sudden) birth of entanglement
The concept of time delayed creation of entanglement by the dissipative
process of spontaneous emission is investigated. A threshold effect for the
creation of entanglement is found that the initially unentangled qubits can be
entangled after a finite time despite the fact that the coherence between the
qubits exists for all times. This delayed creation of entanglement, that we
call sudden birth of entanglement, is opposite to the currently extensively
discussed sudden death of entanglement and is characteristic for transient
dynamics of one-photon entangled states of the system. We determine the
threshold time for the creation of entanglement and find that it is related to
time at which the antisymmetric state remains the only excited state being
populated. It is shown that the threshold time can be controlled by the
distance between the qubits and the direction of initial excitation relative to
the interatomic axis. This effect suggests a new alternative for the study of
entanglement and provides an interesting resource for creation on demand of
entanglement between two qubits.Comment: References added, version accepted for publication in PR
Entanglement induced by spontaneous emission in spatially extended two-atom systems
We investigate the role of the collective antisymmetric state in entanglement
creation by spontaneous emission in a system of two non-overlapping two-level
atoms. We calculate and illustrate graphically populations of the collective
atomic states and the Wootters entanglement measure (concurrence) for two sets
of initial atomic conditions. Our calculations include the dipole-dipole
interaction and a spatial separation between the atoms that the antisymmetric
state of the system is included throughout even for small interatomic
separations. It is shown that spontaneous emission can lead to a transient
entanglement between the atoms even if the atoms were prepared initially in an
unentangled state. We find that the ability of spontaneous emission to create
the transient entanglement relies on the absence of population in the
collective symmetric state of the system. For the initial state of only one
atom excited, the entanglement builds up rapidly in time and reaches a maximum
for the parameter values corresponding roughly to zero population in the
symmetric state. On the other hand, for the initial condition of both atoms
excited, the atoms remain unentangled until the symmetric state is depopulated.
A simple physical interpretation of these results is given in terms of the
diagonal states of the density matrix of the system. We also study entanglement
creation in a system of two non-identical atoms of different transition
frequencies. It is found that the entanglement between the atoms can be
enhanced compared to that for identical atoms, and can decay with two different
time scales resulting from the coherent transfer of the population from the
symmetric to the antisymmetric state. In addition, we find that a decaying
initial entanglement between the atoms can display a revival behaviour.Comment: 14 pages, 6 figure
Entangling two atoms via spontaneous emission
We discuss the creation of entanglement between two two-level atoms in the
dissipative process of spontaneous emission. It is shown that spontaneous
emission can lead to a transient entanglement between the atoms even if the
atoms were prepared initially in an unentangled state. The amount of
entanglement created in the system is quantified by using two different
measures: concurrence and negativity. We find analytical formulas for the
evolution of concurrence and negativity in the system. We also find the
analytical relation between the two measures of entanglement. The system
consists of two two-level atoms which are separated by an arbitrary distance
and interact with each other via the dipole-dipole interaction, and
the antisymmetric state of the system is included throughout, even for small
inter-atomic separations, in contrast to the small sample model. It is shown
that for sufficiently large values of the dipole-dipole interaction initially
the entanglement exhibits oscillatory behaviour with considerable entanglement
in the peaks. For longer times the amount of entanglement is directly related
to the population of the slowly decaying antisymmetric state.Comment: 13 pages, 5 figure
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
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
Food-bridging: a new network construction to unveil the principles of cooking
In this manuscript we propose, analyse, and discuss a possible new principle
behind traditional cuisine: the Food-bridging hypothesis and its comparison
with the food-pairing hypothesis using the same dataset and graphical models
employed in the food-pairing study by Ahn et al. [Scientific Reports, 1:196
(2011)].
The Food-bridging hypothesis assumes that if two ingredients do not share a
strong molecular or empirical affinity, they may become affine through a chain
of pairwise affinities. That is, in a graphical model as employed by Ahn et
al., a chain represents a path that joints the two ingredients, the shortest
path represents the strongest pairwise chain of affinities between the two
ingredients.
Food-pairing and Food-bridging are different hypotheses that may describe
possible mechanisms behind the recipes of traditional cuisines. Food-pairing
intensifies flavour by mixing ingredients in a recipe with similar chemical
compounds, and food-bridging smoothes contrast between ingredients. Both
food-pairing and food-bridging are observed in traditional cuisines, as shown
in this work.
We observed four classes of cuisines according to food-pairing and
food-bridging: (1) East Asian cuisines, at one extreme, tend to avoid
food-pairing as well as food-bridging; and (4) Latin American cuisines, at the
other extreme, follow both principles. For the two middle classes: (2)
Southeastern Asian cuisines, avoid food-pairing and follow food-bridging; and
(3) Western cuisines, follow food-pairing and avoid food-bridging
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
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
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