85 research outputs found
Transfer of quantum states using finite resources
We discuss the problem of transfering a qubit from Alice to Bob using a noisy
quantum channel and only finite resources. As the basic protocol for the
transfer we apply quantum teleportation. It turns out that for a certain
quality of the channel direct teleportation combined with qubit purification is
superior to entanglement purification of the channel. If, however, the quality
of the channel is rather low one should simply apply an estimation-preparation
scheme.Comment: 9 pages RevTeX including 5 figures, replaced with revised version, to
appear in Phys. Rev.
Enhanced Quantum Estimation via Purification
We analyze the estimation of a finite ensemble of quantum bits which have
been sent through a depolarizing channel. Instead of using the depolarized
qubits directly, we first apply a purification step and show that this improves
the fidelity of subsequent quantum estimation. Even though we lose some qubits
of our finite ensemble the information is concentrated in the remaining
purified ones.Comment: 6 pages, including 3 figure
Collective versus local measurements on two parallel or antiparallel spins
We give a complete analysis of covariant measurements on two spins. We
consider the cases of two parallel and two antiparallel spins, and we consider
both collective measurements on the two spins, and measurements which require
only Local Quantum Operations and Classical Communication (LOCC). In all cases
we obtain the optimal measurements for arbitrary fidelities. In particular we
show that if the aim is determine as well as possible the direction in which
the spins are pointing, it is best to carry out measurements on antiparallel
spins (as already shown by Gisin and Popescu), second best to carry out
measurements on parallel spins and worst to be restricted to LOCC measurements.
If the the aim is to determine as well as possible a direction orthogonal to
that in which the spins are pointing, it is best to carry out measurements on
parallel spins, whereas measurements on antiparallel spins and LOCC
measurements are both less good but equivalent.Comment: 4 pages; minor revision
Optimal estimation of quantum dynamics
We construct the optimal strategy for the estimation of an unknown unitary
transformation . This includes, in addition to a convenient
measurement on a probe system, finding which is the best initial state on which
is to act. When , such an optimal strategy can be applied to
estimate simultaneously both the direction and the strength of a magnetic
field, and shows how to use a spin 1/2 particle to transmit information about a
whole coordinate system instead of only a direction in space.Comment: 4 pages, REVTE
Minimal measurements of the gate fidelity of a qudit map
We obtain a simple formula for the average gate fidelity of a linear map
acting on qudits. It is given in terms of minimal sets of pure state
preparations alone, which may be interesting from the experimental point of
view. These preparations can be seen as the outcomes of certain minimal
positive operator valued measures. The connection of our results with these
generalized measurements is briefly discussed
Universality of optimal measurements
We present optimal and minimal measurements on identical copies of an unknown
state of a qubit when the quality of measuring strategies is quantified with
the gain of information (Kullback of probability distributions). We also show
that the maximal gain of information occurs, among isotropic priors, when the
state is known to be pure. Universality of optimal measurements follows from
our results: using the fidelity or the gain of information, two different
figures of merits, leads to exactly the same conclusions. We finally
investigate the optimal capacity of copies of an unknown state as a quantum
channel of information.Comment: Revtex, 5 pages, no figure
Selection rules for J^PC Exotic Hybrid Meson Decay in Large-N_c
The coupling of a neutral hybrid {1,3,5...}^-+ exotic particle (or current)
to two neutral (hybrid) meson particles with the same J^PC and J=0 is proved to
be sub-leading to the usual large-N_c QCD counting. The coupling of the same
exotic particle to certain two - (hybrid) meson currents with the same J^PC and
J=0 is also sub-leading. The decay of a {1,3,5...}^-+ hybrid to eta pi^0, eta'
pi^0, eta' eta, eta(1295) pi^0, pi(1300)^0 pi0, eta(1440) pi^0, a_0(980)^0
sigma or f_0(980) sigma is sub-leading, assuming that these final state
particles are (hybrid) mesons in the limit of large N_c.Comment: 16 pages, LaTeX. Main paper shortened/rewritten and appendices
expanded. Implications for phenomenology of exotic hybrid mesons clarifie
(Field) Symmetrization Selection Rules
QCD and QED exhibit an infinite set of three-point Green's functions that
contain only OZI rule violating contributions, and (for QCD) are subleading in
the large N_c expansion. The Green's functions describe the ``decay'' of a
J^{PC}={1,3,5 ...}^{-+} exotic hybrid meson current to two J=0 (hybrid) meson
currents with identical P and C. We prove that the QCD amplitude for a neutral
hybrid {1,3,5 ...}^{-+} exotic current to create eta pi0 only comes from OZI
rule violating contributions under certain conditions, and is subleading in
N_c.Comment: 20 pages, LaTeX. Two postscript figures. Final published versio
Block Spin Density Matrix of the Inhomogeneous AKLT Model
We study the inhomogeneous generalization of a 1-dimensional AKLT spin chain
model. Spins at each lattice site could be different. Under certain conditions,
the ground state of this AKLT model is unique and is described by the
Valence-Bond-Solid (VBS) state. We calculate the density matrix of a contiguous
block of bulk spins in this ground state. The density matrix is independent of
spins outside the block. It is diagonalized and shown to be a projector onto a
subspace. We prove that for large block the density matrix behaves as the
identity in the subspace. The von Neumann entropy coincides with Renyi entropy
and is equal to the saturated value.Comment: 20 page
Collective vs local measurements in qubit mixed state estimation
We discuss the problem of estimating a general (mixed) qubit state. We give
the optimal guess that can be inferred from any given set of measurements. For
collective measurements and for a large number of copies, we show that the
error in the estimation goes as 1/N. For local measurements we focus on the
simpler case of states lying on the equatorial plane of the Bloch sphere. We
show that standard tomographic techniques lead to an error proportional to
, while with our optimal data processing it is proportional to
.Comment: 4 pages, 1 figure, minor style changes, refs. adde
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