333 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
Negative Binomial States of the Radiation Field and their Excitations are Nonlinear Coherent States
We show that the well-known negative binomial states of the radiation field
and their excitations are nonlinear coherent states. Excited nonlinear coherent
state are still nonlinear coherent states with different nonlinear functions.
We finally give exponential form of the nonlinear coherent states and remark
that the binomial states are not nonlinear coherent states.Comment: 10 pages, no 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
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
Coherent states, displaced number states and Laguerre polynomial states for su(1,1) Lie algebra
The ladder operator formalism of a general quantum state for su(1,1) Lie
algebra is obtained. The state bears the generally deformed oscillator
algebraic structure. It is found that the Perelomov's coherent state is a
su(1,1) nonlinear coherent state. The expansion and the exponential form of the
nonlinear coherent state are given. We obtain the matrix elements of the
su(1,1) displacement operator in terms of the hypergeometric functions and the
expansions of the displaced number states and Laguerre polynomial states are
followed. Finally some interesting su(1,1) optical systems are discussed.Comment: 16 pages, no figures, accepted by Int. J. Mod. Phy.
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.
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
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
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
- …