360 research outputs found
Adaptive Quantum Homodyne Tomography
An adaptive optimization technique to improve precision of quantum homodyne
tomography is presented. The method is based on the existence of so-called null
functions, which have zero average for arbitrary state of radiation. Addition
of null functions to the tomographic kernels does not affect their mean values,
but changes statistical errors, which can then be reduced by an optimization
method that "adapts" kernels to homodyne data. Applications to tomography of
the density matrix and other relevant field-observables are studied in detail.Comment: Latex (RevTex class + psfig), 9 Figs, Submitted to PR
Group Theoretical Quantum Tomography
The paper is devoted to the mathematical foundation of the quantum tomography
using the theory of square-integrable representations of unimodular Lie groups.Comment: 13 pages, no figure, Latex2e. Submitted to J.Math.Phy
Quantum tomography of mesoscopic superpositions of radiation states
We show the feasibility of a tomographic reconstruction of Schr\"{o}dinger
cat states generated according to the scheme proposed by S. Song, C.M. Caves
and B. Yurke [Phys. Rev. A 41, 5261 (1990)]. We present a technique that
tolerates realistic values for quantum efficiency at photodetectors. The
measurement can be achieved by a standard experimental setup.Comment: Submitted to Phys. Rev. Lett.; 4 pages including 6 ps figure
Comment on "Loss-error compensation in quantum-state measurements"
In the two papers [T. Kiss, U. Herzog, and U. Leonhardt, Phys. Rev. A 52,
2433 (1995); U. Herzog, Phys. Rev. A 53, 1245 (1996)] with titles similar to
the one given above, the authors assert that in some cases it is possible to
compensate a quantum efficiency in quantum-state measurements,
violating the lower bound 1/2 proved in a preceding paper [G. M. D'Ariano, U.
Leonhardt and H. Paul, Phys. Rev. A 52, R1801 (1995)]. Here we re-establish the
bound as unsurpassable for homodyning any quantum state, and show how the
proposed loss-compensation method would always fail in a real measurement
outside the allowed region.Comment: 3 pages, RevTeX, 2 figures included, to appear on Phys. Rev. A (April
1998
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
Parameters estimation in quantum optics
We address several estimation problems in quantum optics by means of the
maximum-likelihood principle. We consider Gaussian state estimation and the
determination of the coupling parameters of quadratic Hamiltonians. Moreover,
we analyze different schemes of phase-shift estimation. Finally, the absolute
estimation of the quantum efficiency of both linear and avalanche
photodetectors is studied. In all the considered applications, the Gaussian
bound on statistical errors is attained with a few thousand data.Comment: 11 pages. 6 figures. Accepted on Phys. Rev.
Optimal Non-Universally Covariant Cloning
We consider non-universal cloning maps, namely cloning transformations which
are covariant under a proper subgroup G of the universal unitary group U(d),
where d is the dimension of the Hilbert space H of the system to be cloned. We
give a general method for optimizing cloning for any cost-function. Examples of
applications are given for the phase-covariant cloning (cloning of equatorial
qubits) and for the Weyl-Heisenberg group (cloning of "continuous variables").Comment: 6 page
Exploiting quantum parallelism of entanglement for a complete experimental quantum characterization of a single qubit device
We present the first full experimental quantum tomographic characterization
of a single-qubit device achieved with a single entangled input state. The
entangled input state plays the role of all possible input states in quantum
parallel on the tested device. The method can be trivially extended to any
n-qubits device by just replicating the whole experimental setup n times.Comment: 4 pages in revtex4 with 4 eps figure
Characterising a universal cloning machine by maximum-likelihood estimation
We apply a general method for the estimation of completely positive maps to
the 1-to-2 universal covariant cloning machine. The method is based on the
maximum-likelihood principle, and makes use of random input states, along with
random projective measurements on the output clones. The downhill simplex
algorithm is applied for the maximisation of the likelihood functional.Comment: 5 pages, 2 figure
Quantum Circuits Architecture
We present a method for optimizing quantum circuits architecture. The method
is based on the notion of "quantum comb", which describes a circuit board in
which one can insert variable subcircuits. The method allows one to efficiently
address novel kinds of quantum information processing tasks, such as
storing-retrieving, and cloning of channels.Comment: 10 eps figures + Qcircuit.te
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