535 research outputs found

    Quantum Binary Decision for Driven Harmonic Oscillator

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    We address the problem of determining whether or not a harmonic oscillator has been perturbed by an external force. Quantum detection and estimation theory has been used in devising optimum measurement schemes. Detection probability has been evaluated for different initial state preparations of oscillator. The corresponding lower bounds on minimum detectable perturbation intensity has been evaluated and a general bound for random phase perturbation has been also induced.Comment: Latex 5 figs -- More info at http://enterprise.pv.infn.it/~pari

    Unitary local invariance

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    We address unitary local (UL) invariance of bipartite pure states. Given a bipartite state Ψ>>=ijψiji>1j>2|\Psi>>=\sum_{ij} \psi_{ij}\: |i>_1\otimes |j>_2 the complete characterization of the class of local unitaries U1U2U_1\otimes U_2 for which U1U2Ψ>>=Ψ>>U_1\otimes U_2 |\Psi>>=|\Psi>> is obtained.The two relevant parameters are the rank of the matrix Ψ\Psi, [Ψ]ij=ψij[\Psi]_{ij}=\psi_{ij}, and the number of its equal singular values, {\em i.e.} the degeneracy of the eigenvalues of the partial traces of Ψ>>|\Psi>>.Comment: Revised version. Accepted for publication Int. J. Quant. In