21 research outputs found
Entanglement transfer between bipartite systems
The problem of a controlled transfer of an entanglement initially encoded
into two two-level atoms that are successively sent through two single-mode
cavities is investigated. The atoms and the cavity modes form a four qubit
system and we demonstrate under which conditions the initial entanglement
encoded into the atoms can be completely transferred to other pairs of qubits.
We find that in the case of a nonzero detuning between the atomic transition
frequencies and the cavity mode frequencies, no complete transfer of the
initial entanglement is possible to any of the other pairs of qubits. In the
case of exact resonance and equal coupling strengths of the atoms to the cavity
modes, an initial maximally entangled state of the atoms can be completely
transferred to the cavity modes. The complete transfer of the entanglement is
restricted to the cavity modes only with the transfer to the other pairs being
limited to up to 50%. We have found that the complete transfer of an initial
entanglement to other pairs of qubits may take place if the initial state is
not the maximally entangled state and the atoms couple to the cavity modes with
unequal strengths. Depending on the ratio between the coupling strengths, the
optimal entanglement can be created between the atoms and one of the cavity
modes.Comment: 3 figures. Oral talk presented in CEWQO 18, Madrid 201
Entanglement Dynamics in Two-Qubit Open System Interacting with a Squeezed Thermal Bath via Quantum Nondemolition interaction
We analyze the dynamics of entanglement in a two-qubit system interacting
with an initially squeezed thermal environment via a quantum nondemolition
system-reservoir interaction, with the system and reservoir assumed to be
initially separable. We compare and contrast the decoherence of the two-qubit
system in the case where the qubits are mutually close-by (`collective regime')
or distant (`localized regime') with respect to the spatial variation of the
environment. Sudden death of entanglement (as quantified by concurrence) is
shown to occur in the localized case rather than in the collective case, where
entanglement tends to `ring down'. A consequence of the QND character of the
interaction is that the time-evolved fidelity of a Bell state never falls below
, a fact that is useful for quantum communication applications like
a quantum repeater. Using a novel quantification of mixed state entanglement,
we show that there are noise regimes where even though entanglement vanishes,
the state is still available for applications like NMR quantum computation,
because of the presence of a pseudo-pure component.Comment: 17 pages, 9 figures, REVTeX
Entanglement sudden birth of two trapped ions interacting with a time-dependent laser field
We explore and develop the mathematics of the two multi-level ions. In
particular, we describe some new features of quantum entanglement in two
three-level trapped ions confined in a one-dimensional harmonic potential,
allowing the instantaneous position of the center-of-mass motion of the ions to
be explicitly time-dependent. By solving the exact dynamics of the system, we
show how survivability of the quantum entanglement is determined by a specific
choice of the initial state settings.Comment: 13 pages, 4 figure
Dual-probe decoherence microscopy: Probing pockets of coherence in a decohering environment
We study the use of a pair of qubits as a decoherence probe of a non-trivial
environment. This dual-probe configuration is modelled by three
two-level-systems which are coupled in a chain in which the middle system
represents an environmental two-level-system (TLS). This TLS resides within the
environment of the qubits and therefore its coupling to perturbing fluctuations
(i.e. its decoherence) is assumed much stronger than the decoherence acting on
the probe qubits. We study the evolution of such a tripartite system including
the appearance of a decoherence-free state (dark state) and non-Markovian
behaviour. We find that all parameters of this TLS can be obtained from
measurements of one of the probe qubits. Furthermore we show the advantages of
two qubits in probing environments and the new dynamics imposed by a TLS which
couples to two qubits at once.Comment: 29 pages, 10 figure
Dynamics of impurity, local and non-local information for two non identical qubits
From the separability point of view the problem of two atoms interact with a
single cavity mode is investigated. The density matrix is calculated and used
to discuss the entanglement and to examine the dynamics of the local and
non-local information. Our examination concentrated on the variation in the
mean photon number and the ratio of the coupling parameters. Furthermore, we
have also assumed that the atomic system is initially in the ground states as
well as in the intermediate states. It has been shown that the local
information is transferred to non-local information when the impurity of one
qubit or both is maximum
Decoherence and entanglement degradation of a qubit-qutrit system in non-inertial frames
We study the effect of decoherence on a qubit-qutrit system under the
influence of global, local and multilocal decoherence in non-inertial frames.
We show that the entanglement sudden death can be avoided in non-inertial
frames in the presence of amplitude damping, depolarizing and phase damping
channels. However, degradation of entanglement is seen due to Unruh effect. It
is shown that for lower level of decoherence, the depolarizing channel degrades
the entanglement more heavily as compared to the amplitude damping and phase
damping channels. However, for higher values of decoherence parameters,
amplitude damping channel heavily degrades the entanglement of the hybrid
system. Further more, no ESD is seen for any value of Rob's acceleration.Comment: 16 pages, 5 .eps figures, 1 table; Quantum Information Processing,
published online, 5 July, 201
Quantum entanglement and disentanglement of multi-atom systems
We present a review of recent research on quantum entanglement, with special
emphasis on entanglement between single atoms, processing of an encoded
entanglement and its temporary evolution. Analysis based on the density matrix
formalism are described. We give a simple description of the entangling
procedure and explore the role of the environment in creation of entanglement
and in disentanglement of atomic systems. A particular process we will focus on
is spontaneous emission, usually recognized as an irreversible loss of
information and entanglement encoded in the internal states of the system. We
illustrate some certain circumstances where this irreversible process can in
fact induce entanglement between separated systems. We also show how
spontaneous emission reveals a competition between the Bell states of a two
qubit system that leads to the recently discovered "sudden" features in the
temporal evolution of entanglement. An another problem illustrated in details
is a deterministic preparation of atoms and atomic ensembles in long-lived
stationary squeezed states and entangled cluster states. We then determine how
to trigger the evolution of the stable entanglement and also address the issue
of a steered evolution of entanglement between desired pairs of qubits that can
be achieved simply by varying the parameters of a given system.Comment: Review articl
Photon wave functions, wave-packet quantization of light, and coherence theory
The monochromatic Dirac and polychromatic Titulaer-Glauber quantized field
theories (QFTs) of electromagnetism are derived from a photon-energy wave
function in much the same way that one derives QFT for electrons, that is, by
quantization of a single-particle wave function. The photon wave function and
its equation of motion are established from the Einstein energy-momentum-mass
relation, assuming a local energy density. This yields a theory of photon wave
mechanics (PWM). The proper Lorentz-invariant single-photon scalar product is
found to be non-local in coordinate space, and is shown to correspond to
orthogonalization of the Titulaer-Glauber wave-packet modes. The wave functions
of PWM and mode functions of QFT are shown to be equivalent, evolving via
identical equations of motion, and completely describe photonic states. We
generalize PWM to two or more photons, and show how to switch between the PWM
and QFT viewpoints. The second-order coherence tensors of classical coherence
theory and the two-photon wave functions are shown to propagate equivalently.
We give examples of beam-like states, which can be used as photon wave
functions in PWM, or modes in QFT. We propose a practical mode converter based
on spectral filtering to convert between wave packets and their corresponding
biorthogonal dual wave packets.Comment: 34 pages, 3 figures, minor correction