1,071 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
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
Planar domain walls in black hole spacetimes
We investigate the behaviour of low-mass, planar domain walls in the
so-called model of the scalar field on the Schwarzschild and Kerr
backgrounds. We focus on a transit of a domain wall through a black hole and
solve numerically the equations of motion for a range of parameters of the
domain wall and the black hole. We observe a behavior resembling an occurrence
of ringing modes. Perturbations of domain walls vanish during latter evolution,
suggesting their stability against a passage through the black hole. The
results obtained for Kerr and Reissner-Nordstr\"om black holes are also
compared.Comment: 13 pages, 8 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
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