948 research outputs found
A Deterministic and Nondestructively-Verifiable Photon Number Source
Full text linkWe present a deterministic approach based on continuous measurement and
real-time quantum feedback control to prepare arbitrary photon number states of
a cavity mode. The procedure passively monitors the number state actually
achieved in each feedback stabilized measurement trajectory, thus providing a
nondestructively verifiable photon source. The feasibility of a possible cavity
QED implementation in the many-atom good-cavity coupling regime is analyzed
Stability of macroscopic entanglement under decoherence
Full text linkWe investigate the lifetime of macroscopic entanglement under the influence
of decoherence. For GHZ-type superposition states we find that the lifetime
decreases with the size of the system (i.e. the number of independent degrees
of freedom) and the effective number of subsystems that remain entangled
decreases with time. For a class of other states (e.g. cluster states),
however, we show that the lifetime of entanglement is independent of the size
of the system.Comment: 5 pages, 1 figur
Multiparticle entanglement purification for graph states
Full text linkWe introduce a class of multiparticle entanglement purification protocols
that allow us to distill a large class of entangled states. These include
cluster states, GHZ states and various error correction codes all of which
belong to the class of two-colorable graph states. We analyze these schemes
under realistic conditions and observe that they are scalable, i.e. the
threshold value for imperfect local operations does not depend on the number of
parties for many of these states. When compared to schemes based on bipartite
entanglement purification, the protocol is more efficient and the achievable
quality of the purified states is larger. As an application we discuss an
experimental realization of the protocol in optical lattices which allows one
to purify cluster states.Comment: 4 pages, 2 figures; V2: some typos corrected; V3: published versio
Entanglement properties of multipartite entangled states under the influence of decoherence
Full text linkWe investigate entanglement properties of multipartite states under the
influence of decoherence. We show that the lifetime of (distillable)
entanglement for GHZ-type superposition states decreases with the size of the
system, while for a class of other states -namely all graph states with
constant degree- the lifetime is independent of the system size. We show that
these results are largely independent of the specific decoherence model and are
in particular valid for all models which deal with individual couplings of
particles to independent environments, described by some quantum optical master
equation of Lindblad form. For GHZ states, we derive analytic expressions for
the lifetime of distillable entanglement and determine when the state becomes
fully separable. For all graph states, we derive lower and upper bounds on the
lifetime of entanglement. To this aim, we establish a method to calculate the
spectrum of the partial transposition for all mixed states which are diagonal
in a graph state basis. We also consider entanglement between different groups
of particles and determine the corresponding lifetimes as well as the change of
the kind of entanglement with time. This enables us to investigate the behavior
of entanglement under re-scaling and in the limit of large (infinite) number of
particles. Finally we investigate the lifetime of encoded quantum superposition
states and show that one can define an effective time in the encoded system
which can be orders of magnitude smaller than the physical time. This provides
an alternative view on quantum error correction and examples of states whose
lifetime of entanglement (between groups of particles) in fact increases with
the size of the system.Comment: 27 pages, 11 figure
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