269 research outputs found
Bell Measurements and Observables
A general matrix approach to study entangled states is presented, based on
operator completeness relations. Bases of unitary operators are considered,
with focus on irreducible representations of groups. Bell measurements for
teleportation are considered, and robustness of teleportation to various kinds
of non idealities is shown.Comment: 11 pages. Elsart styl
Optical von Neumann measurement
We present an optical scheme that realizes the standard von Neumann
measurement model, providing an indirect measurement of a quadrature of the
field with controllable Gaussian state-reduction. The scheme is made of simple
optical elements, as laser sources, beam splitters, and phase sensitive
amplifiers, along with a feedback mechanism that uses a Pockels cell. We show
that the von Neumann measurement is achieved without the need of working in a
ultra-short pulsed regime.Comment: One latex figure. Accepted on Phys. Lett.
On the general problem of quantum phase estimation
The problem of estimating a generic phase-shift experienced by a quantum
state is addressed for a generally degenerate phase shift operator. The optimal
positive operator-valued measure is derived along with the optimal input state.
Two relevant examples are analyzed: i) a multi-mode phase shift operator for
multipath interferometry; ii) the two mode heterodyne phase detection.Comment: 11 pages. Elsart package use
Probability-fidelity tradeoffs for targeted quantum operations
We present probability-fidelity tradeoffs for a varying quantum operation
with fixed input-output states and for a varying inversion of a fixed quantum
operation.Comment: 6 pages, 5 figure
Optimal realization of the transposition maps
We solve the problem of achieving the optimal physical approximation of the
transposition for pure states of arbitrary quantum systems for finite and
infinite dimensions. A unitary realization is also given for any finite
dimension, which provides the optimal quantum cloning map of the ancilla as
well.Comment: 10 pages. No figures. Elsart styl
Quantum inference of states and processes
The maximum-likelihood principle unifies inference of quantum states and
processes from experimental noisy data. Particularly, a generic quantum process
may be estimated simultaneously with unknown quantum probe states provided that
measurements on probe and transformed probe states are available. Drawbacks of
various approximate treatments are considered.Comment: 7 pages, 4 figure
Renormalized quantum tomography
The core of quantum tomography is the possibility of writing a generally
unbounded complex operator in form of an expansion over operators that are
generally nonlinear functions of a generally continuous set of spectral
densities--the so-called "quorum" of observables. The expansion is generally
non unique, the non unicity allowing further optimization for given criteria.
The mathematical problem of tomography is thus the classification of all such
operator expansions for given (suitably closed) linear spaces of unbounded
operators--e.g. Banach spaces of operators with an appropriate norm. Such
problem is a difficult one, and remains still open, involving the theory of
general basis in Banach spaces, a still unfinished chapter of analysis. In this
paper we present new nontrivial operator expansions for the quorum of
quadratures of the harmonic oscillator, and introduce a first very preliminary
general framework to generate new expansions based on the Kolmogorov
construction. The material presented in this paper is intended to be helpful
for the solution of the general problem of quantum tomography in infinite
dimensions, which corresponds to provide a coherent mathematical framework for
operator expansions over functions of a continuous set of spectral densities.Comment: 23 pages, no figure
Optimal local discrimination of two multipartite pure states
In a recent paper, Walgate et. al. demonstrated that any two orthogonal
multipartite pure states can be optimally distinguished using only local
operations. We utilise their result to show that this is true for any two
multiparty pure states, in the sense of inconclusive discrimination. There are
also certain regimes of conclusive discrimination for which the same also
applies, although we can only conjecture that the result is true for all
conclusive regimes. We also discuss a class of states that can be distinguished
locally according to any discrimination measure, as they can be locally
recreated in the hands of one party. A consequence of this is that any two
maximally entangled states can always be optimally discriminated locally,
according to any figure of merit.Comment: Published version, results unchanged, although errors in the last
proof have been correcte
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