37 research outputs found
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
Quantum non-demolition measurement saturates fidelity trade-off
A general quantum measurement on an unknown quantum state enables us to
estimate what the state originally was. Simultaneously, the measurement has a
destructive effect on a measured quantum state which is reflected by the
decrease of the output fidelity. We show for any -level system that quantum
non-demolition (QND) measurement controlled by a suitably prepared ancilla is a
measurement in which the decrease of the output fidelity is minimal. The ratio
between the estimation fidelity and the output fidelity can be continuously
controlled by the preparation of the ancilla. Different measurement strategies
on the ancilla are also discussed. Finally, we propose a feasible scheme of
such a measurement for atomic and optical 2-level systems based on basic
controlled-NOT gate.Comment: 5 pages, 2 figure
Minimal disturbance measurement for coherent states is non-Gaussian
In standard coherent state teleportation with shared two-mode squeezed vacuum
(TMSV) state there is a trade-off between the teleportation fidelity and the
fidelity of estimation of the teleported state from results of the Bell
measurement. Within the class of Gaussian operations this trade-off is optimal,
i.e. there is not a Gaussian operation which would give for a given output
fidelity a larger estimation fidelity. We show that this trade-off can be
improved by up to 2.77% if we use a suitable non-Gaussian operation. This
operation can be implemented by the standard teleportation protocol in which
the shared TMSV state is replaced with a suitable non-Gaussian entangled state.
We also demonstrate that this operation can be used to enhance the transmission
fidelity of a certain noisy channel.Comment: submitted to Physical Review A, new results added, 7 pages, 4 figure
Optimal partial estimation of quantum states from several copies
We derive analytical formula for the optimal trade-off between the mean
estimation fidelity and the mean fidelity of the qubit state after a partial
measurement on N identically prepared qubits. We also conjecture analytical
expression for the optimal fidelity trade-off in case of a partial measurement
on N identical copies of a d-level system.Comment: 5 pages, 2 figures, RevTeX
Mixed state localizable entanglement for continuous variables
We investigate localization of entanglement of multipartite mixed Gaussian
states into a pair of modes by local Gaussian measurements on the remaining
modes and classical communication. We provide a detailed proof that for
arbitrary symmetric Gaussian state maximum entanglement can be localized by
homodyne detection of either amplitude or phase quadrature on each mode. We
then consider arbitrary mixed three-mode Gaussian states and show that the
optimal Gaussian measurement on one mode yielding maximum entanglement among
the other two modes can be determined by calculating roots of a high-order
polynomial. Finally, we discuss localization of entanglement with single-photon
detection.Comment: 11 pages, 3 figure