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

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