94 research outputs found
Weak Measurements with Entangled Probes
Encoding the imaginary part of a weak value onto an initially entangled probe
can modify its entanglement content - provided the probe observable can
distinguish between states of different entropies. Apart from fundamental
interest, this result illustrates the utility of the imaginary weak value as a
calculational tool in certain entanglement concentration protocols.Comment: 4 pages, single diagra
Quantum discord from system-environment correlations
In an initially uncorrelated mixed separable bi-partite system, quantum
correlations can emerge under the action of a local measurement or local noise
[A. Streltsov, H. Kampermann, and D. Bru\ss{}, Phys. Rev. Lett. 107, 170502
(2011)]. We analyze this counter-intuitive phenomenon using quantum discord as
a quantifier. We then relate changes in quantum discord to system-environment
correlations between the system in a mixed state and some purifying
environmental mode using the Koashi-Winter inequality. On this basis, we
suggest an interpretation of discord as a byproduct of transferring
entanglement and correlations around the different subsystems of a global pure
state.Comment: 6 pages, submitted to the special issue of Physica Scripta on CEWQO
201
Procrustean entanglement concentration of continuous variable states of light
We propose a Procrustean entanglement concentration scheme for continuous
variable states inspired by the scheme proposed in Fiurasek et. al. Phys. Rev.
A 67, 022304, (2003). We show that the eight-port homodyne measurement of
Fiurasek et. al. Phys. Rev. A 67, 022304, (2003) can be replaced by a balanced
homodyne measurement with the advantage of providing a success criterion that
allows Alice and Bob to determine if entanglement concentration was achieved.
In addition, it facilitates a straightforward and feasible experimental
implementation.Comment: 6 pages, 3 figures; Typos corrected, References added and conclusion
extended, Accepted for publication in Physical Review
Cross-Kerr interaction in a four-level atomic system
We derive the form of the cross-Kerr interaction in a four-level atomic
system in the N-configuration. We use time-independent perturbation theory to
calculate the eigenenergies and eigenstates of the Schrodinger equation for the
system. The system is considered as a perturbation of a Raman resonant
three-level lambda scheme for which exact solutions are known. We show that
within the strong control field limit the cross-Kerr interaction can arise
between two weak probe fields. The strength of this nonlinear coupling is
several orders of magnitude larger than that achievable using optical fibres.Comment: 5 pages, resubmitted to Physical Review A with clarified style and
correction to Fig
Distribution of continuous-variable entanglement by separable Gaussian states
Entangling two systems at distant locations using a {\it separable} mediating
ancilla is a counterintuitive phenomenon proposed for qubits by T. Cubitt {\it
et al}. [Phys. Rev. Lett. {\bf 91}, 037902 (2003)]. We show that such
entanglement distribution is possible with Gaussian states, using a certain
three-mode fully separable mixed Gaussian state and linear optics elements
readily available in experiments. Two modes of the state become entangled by
sequentially mixing them on two beam splitters, while the third one remains
separable in all stages of the protocol
Composite Cluster States and Alternative Architectures for One- Way Quantum Computation
We propose a new architecture for the measurement-based quantum computation
model. The new design relies on small composite light-atom primary clusters.
These are then assembled into cluster arrays using ancillary light modes and
the actual computation is run on such a cellular cluster. We show how to create
the primary clusters, which are Gaussian cluster states composed of both light
and atomic modes. These are entangled via QND interactions and beamsplitters
and the scheme is well described within the continuous-variable covariance
matrix formalism.Comment: arXiv admin note: text overlap with arXiv:1007.040
Quantum steering as a resource for secure tripartite Quantum State Sharing
Quantum State Sharing (QSS) is a protocol by which a (secret) quantum state
may be securely split, shared between multiple potentially dishonest players,
and reconstructed. Crucially the players are each assumed to be dishonest, and
so QSS requires that only a collaborating authorised subset of players can
access the original secret state; any dishonest unauthorised conspiracy cannot
reconstruct it. We analyse a QSS protocol involving three untrusted players and
demonstrate that quantum steering is the required resource which enables the
protocol to proceed securely. We analyse the level of steering required to
share any single-mode Gaussian secret which enables the states to be shared
with the optimal use of resources.Comment: 8 pages, 3 figure
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