4,602 research outputs found
Comment on "Classical interventions in quantum systems II. Relativistic invariance"
In a recent paper [Phys. Rev. A 61, 022117 (2000)], quant-ph/9906034, A.
Peres argued that quantum mechanics is consistent with special relativity by
proposing that the operators that describe time evolution do not need to
transform covariantly, although the measurable quantities need to transform
covariantly. We discuss the weaknesses of this proposal.Comment: 4 pages, to appear in Phys. Rev.
Minimal optimal generalized quantum measurements
Optimal and finite positive operator valued measurements on a finite number
of identically prepared systems have been presented recently. With physical
realization in mind we propose here optimal and minimal generalized quantum
measurements for two-level systems.
We explicitly construct them up to N=7 and verify that they are minimal up to
N=5. We finally propose an expression which gives the size of the minimal
optimal measurements for arbitrary .Comment: 9 pages, Late
Coronal loop hydrodynamics. The solar flare observedon November 12 1980 revisited: the UV line emission
We revisit a well-studied solar flare whose X-ray emission originating from a
simple loop structure was observed by most of the instruments on board SMM on
November 12 1980. The X-ray emission of this flare, as observed with the XRP,
was successfully modeled previously. Here we include a detailed modeling of the
transition region and we compare the hydrodynamic results with the UVSP
observations in two EUV lines, measured in areas smaller than the XRP rasters,
covering only some portions of the flaring loop (the top and the foot-points).
The single loop hydrodynamic model, which fits well the evolution of coronal
lines (those observed with the XRP and the \FeXXI 1354.1 \AA line observed with
the UVSP) fails to model the flux level and evolution of the \OV 1371.3 \AA
line.Comment: A&A, in press, 6 pages, 5 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
A geometric proof of the Kochen-Specker no-go theorem
We give a short geometric proof of the Kochen-Specker no-go theorem for
non-contextual hidden variables models. Note added to this version: I
understand from Jan-Aake Larsson that the construction we give here actually
contains the original Kochen-Specker construction as well as many others (Bell,
Conway and Kochen, Schuette, perhaps also Peres).Comment: This paper appeared some years ago, before the author was aware of
quant-ph. It is relevant to recent developments concerning Kochen-Specker
theorem
A variant of Peres-Mermin proof for testing noncontextual realist models
For any state in four-dimensional system, the quantum violation of an
inequality based on the Peres-Mermin proof for testing noncontextual realist
models has experimentally been corroborated. In the Peres-Mermin proof, an
array of nine holistic observables for two two-qubit system was used. We, in
this letter, present a new symmetric set of observables for the same system
which also provides a contradiction of quantum mechanics with noncontextual
realist models in a state-independent way. The whole argument can also be cast
in the form of a new inequality that can be empirically tested.Comment: 3 pages, To be published in Euro. Phys. Let
Charge and Spin Transport in the One-dimensional Hubbard Model
In this paper we study the charge and spin currents transported by the
elementary excitations of the one-dimensional Hubbard model. The corresponding
current spectra are obtained by both analytic methods and numerical solution of
the Bethe-ansatz equations. For the case of half-filling, we find that the
spin-triplet excitations carry spin but no charge, while charge -spin
triplet excitations carry charge but no spin, and both spin-singlet and charge
-spin-singlet excitations carry neither spin nor charge currents.Comment: 24 pages, 14 figure
Influence of detector motion in entanglement measurements with photons
We investigate how the polarization correlations of entangled photons
described by wave packets are modified when measured by moving detectors. For
this purpose, we analyze the Clauser-Horne-Shimony-Holt Bell inequality as a
function of the apparatus velocity. Our analysis is motivated by future
experiments with entangled photons designed to use satellites. This is a first
step towards the implementation of quantum information protocols in a global
scale
Communication of Spin Directions with Product States and Finite Measurements
Total spin eigenstates can be used to intrinsically encode a direction, which
can later be decoded by means of a quantum measurement. We study the optimal
strategy that can be adopted if, as is likely in practical applications, only
product states of -spins are available. We obtain the asymptotic behaviour
of the average fidelity which provides a proof that the optimal states must be
entangled. We also give a prescription for constructing finite measurements for
general encoding eigenstates.Comment: 4 pages, minor changes, version to appear in PR
On separability of quantum states and the violation of Bell-type inequalities
In contrast to the wide-spread opinion that any separable quantum state
satisfies every classical probabilistic constraint, we present a simple example
where a separable quantum state does not satisfy the original Bell inequality
although the latter inequality, in its perfect correlation form, is valid for
all joint classical measurements. In a very general setting, we discuss
inequalities for joint experiments upon a bipartite quantum system in a
separable state. We derive quantum analogues of the original Bell inequality
and specify the conditions sufficient for a separable state to satisfy the
original Bell inequality. We introduce the extended CHSH inequality and prove
that, for any separable quantum state, this inequality holds for a variety of
linear combinations.Comment: 13 pages, extended versio
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