Chiara Macchiavello

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By R. Cleve1, A. Ekert2, C. Macchiavello2,3 and M. Mosca2,4 2 Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. 1 Department of Computer Science, University of Calgary Calgary, Alberta, Canada T2N 1N4. 3 I.S.I. Foundation, Villa Gualino, Viale Settimio Severo 65, 1033 Torino, Italy. 4 Mathematical Institute,(More)
Existing quantum cryptographic schemes are not, as they stand, operable in the presence of noise on the quantum communication channel. Although they become operable if they are supplemented by classical privacy-amplification techniques, the resulting schemes are difficult to analyse and have not been proved secure. We introduce the concept of quantum(More)
B y R. Cleve1, A. Ekert2, C. Macchiavello2,3 and M. Mosca2,4 2 Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. 1 Department of Computer Science, University of Calgary Calgary, Alberta, Canada T2N 1N4. 3 I.S.I. Foundation, Villa Gualino, Viale Settimio Severo 65, 1033 Torino, Italy. 4 Mathematical(More)
We propose a method for the stabilization of quantum computations (including quantum state storage). The method is based on the operation of projection into SYM, the symmetric subspace of the full state space of R redundant copies of the computer. We describe an efficient algorithm and quantum network effecting SYM–projection and discuss the stabilizing(More)
We establish the best possible approximation to a perfect quantum cloning machine which produces two clones out of a single input. We analyze both universal and state-dependent cloners. The maximal fidelity of cloning is shown to be 5/6 for universal cloners. It can be achieved either by a special unitary evolution or by a novel teleportation scheme. We(More)
Quantum computers use the quantum interference of different computational paths to enhance correct outcomes and suppress erroneous outcomes of computations. In effect, they follow the same logical paradigm as (multi-particle) interferometers. We show how most known quantum algorithms, including quantum algorithms for factorising and counting, may be cast in(More)
We propose a method for the stabilisation of quantum computations (including quantum state storage). The method is based on the operation of projection into SYM, the symmetric subspace of the full state space of R redundant copies of the computer. We describe an efficient algorithm and quantum network effecting SYM–projection and discuss the stabilising(More)
We address the problem of estimating the phase phi given N copies of the phase-rotation gate uphi. We consider, for the first time, the optimization of the general case where the circuit consists of an arbitrary input state, followed by any arrangement of the N phase rotations interspersed with arbitrary quantum operations, and ending with a general(More)
We report on the first experimental realization of an entanglement witness, a method to detect entanglement with few local measurements. The present demonstration has been performed with polarized photons in Werner states, a well-known family of mixed states that can be either separable or nonseparable. The Werner states are generated by a novel high(More)