581 research outputs found
Dynamical creation of entanglement by homodyne-mediated feedback
For two two-level atoms coupled to a single-mode cavity field that is driven
and heavily damped, the steady-state can be entangled by shining an
un-modulated driving laser on the system [S.Schneider, G. J. Milburn Phys. Rev
A 65, 042107, 2002]. We present a scheme to significantly increase the
steady-state entanglement by using homodyne-mediated feedback, in which the
driving laser is modulated by the homodyne photocurrent derived from the cavity
output. Such feedback can increase the nonlinear response to both the
decoherence process of the two-qubit system and the coherent evolution of
individual qubits. We present the properties of the entangled states using the
SO(3) Q function.Comment: 8 page
Finite-time destruction of entanglement and non-locality by environmental influences
Entanglement and non-locality are non-classical global characteristics of
quantum states important to the foundations of quantum mechanics. Recent
investigations have shown that environmental noise, even when it is entirely
local in influence, can destroy both of these properties in finite time despite
giving rise to full quantum state decoherence only in the infinite time limit.
These investigations, which have been carried out in a range of theoretical and
experimental situations, are reviewed here.Comment: 27 pages, 6 figures, review article to appear in Foundations of
Physic
Experimental investigation of the dynamics of entanglement: Sudden death, complementarity, and continuous monitoring of the environment
We report on an experimental investigation of the dynamics of entanglement
between a single qubit and its environment, as well as for pairs of qubits
interacting independently with individual environments, using photons obtained
from parametric down-conversion. The qubits are encoded in the polarizations of
single photons, while the interaction with the environment is implemented by
coupling the polarization of each photon with its momentum. A convenient Sagnac
interferometer allows for the implementation of several decoherence channels
and for the continuous monitoring of the environment. For an
initially-entangled photon pair, one observes the vanishing of entanglement
before coherence disappears. For a single qubit interacting with an
environment, the dynamics of complementarity relations connecting single-qubit
properties and its entanglement with the environment is experimentally
determined. The evolution of a single qubit under continuous monitoring of the
environment is investigated, demonstrating that a qubit may decay even when the
environment is found in the unexcited state. This implies that entanglement can
be increased by local continuous monitoring, which is equivalent to
entanglement distillation. We also present a detailed analysis of the transfer
of entanglement from the two-qubit system to the two corresponding
environments, between which entanglement may suddenly appear, and show
instances for which no entanglement is created between dephasing environments,
nor between each of them and the corresponding qubit: the initial two-qubit
entanglement gets transformed into legitimate multiqubit entanglement of the
Greenberger-Horne-Zeilinger (GHZ) type.Comment: 15 pages, 14 figures; only .ps was working, now .pdf is also
availabl
A highly efficient single photon-single quantum dot interface
Semiconductor quantum dots are a promising system to build a solid state
quantum network. A critical step in this area is to build an efficient
interface between a stationary quantum bit and a flying one. In this chapter,
we show how cavity quantum electrodynamics allows us to efficiently interface a
single quantum dot with a propagating electromagnetic field. Beyond the well
known Purcell factor, we discuss the various parameters that need to be
optimized to build such an interface. We then review our recent progresses in
terms of fabrication of bright sources of indistinguishable single photons,
where a record brightness of 79% is obtained as well as a high degree of
indistinguishability of the emitted photons. Symmetrically, optical
nonlinearities at the very few photon level are demonstrated, by sending few
photon pulses at a quantum dot-cavity device operating in the strong coupling
regime. Perspectives and future challenges are briefly discussed.Comment: to appear as a book chapter in a compilation "Engineering the
Atom-Photon Interaction" published by Springer in 2015, edited by A.
Predojevic and M. W. Mitchel
Dynamics of quantum correlations in two-qubit systems within non-Markovian environments
Knowledge of the dynamical behavior of correlations with no classical
counterpart, like entanglement, nonlocal correlations and quantum discord, in
open quantum systems is of primary interest because of the possibility to
exploit these correlations for quantum information tasks. Here we review some
of the most recent results on the dynamics of correlations in bipartite systems
embedded in non-Markovian environments that, with their memory effects,
influence in a relevant way the system dynamics and appear to be more
fundamental than the Markovian ones for practical purposes. Firstly, we review
the phenomenon of entanglement revivals in a two-qubit system for both
independent environments and a common environment. We then consider the
dynamics of quantum discord in non-Markovian dephasing channel and briefly
discuss the occurrence of revivals of quantum correlations in classical
environments.Comment: 20 pages, 4 figures. Review article, in press in Int. J. Mod. Phys.
B, special issue "Classical Vs Quantum correlations in composite systems",
edited by L. Amico, S. Bose, V. Korepin and V. Vedra
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