663 research outputs found
Complementarity and Information in "Delayed-choice for entanglement swapping"
Building on Peres's idea of "Delayed-choice for extanglement swapping" we
show that even the degree to which quantum systems were entangled can be
defined after they have been registered and may even not exist any more. This
does not arise as a paradox if the quantum state is viewed as just a
representative of information. Moreover such a view gives a natural
quantification of the complementarity between the measure of information about
the input state for teleportation and the amount of entanglement of the
resulting swapped entangled state.Comment: 5 pages, 2 figures, submitted to the special issue of Foundation of
Physics in honor of Asher Peres' 70th birthda
Nonclassicality of pure two-qutrit entangled states
We report an exhaustive numerical analysis of violations of local realism by
two qutrits in all possible pure entangled states. In Bell type experiments we
allow any pairs of local unitary U(3) transformations to define the measurement
bases. Surprisingly, Schmidt rank-2 states, resembling pairs of maximally
entangled qubits, lead to the most noise-robust violations of local realism.
The phenomenon seems to be even more pronounced for four and five dimensional
systems, for which we tested a few interesting examples.Comment: 6 pages, journal versio
Operationally Invariant Information in Quantum Measurements
A new measure of information in quantum mechanics is proposed which takes
into account that for quantum systems the only feature known before an
experiment is performed are the probabilities for various events to occur. The
sum of the individual measures of information for mutually complementary
observations is invariant under the choice of the particular set of
complementary observations and conserved if there is no information exchange
with an environment. That operational quantum information invariant results in
N bits of information for a system consisting of N qubits.Comment: 4 pages, 1 figur
Local Conversion of Greenberger-Horne-Zeilinger States to Approximate W States
Genuine 3-qubit entanglement comes in two different inconvertible types
represented by the Greenberger-Horne-Zeilinger (GHZ) state and the W state. We
describe a specific method based on local positive operator valued measures and
classical communication that can convert the ideal N-qubit GHZ state to a state
arbitrarily close to the ideal N-qubit W state. We then experimentally
implement this scheme in the 3-qubit case and characterize the input and the
final state using 3-photon quantum state tomography.Comment: 4 pages, 3 figure
The puzzlingly large Ca II triplet absorption in dwarf elliptical galaxies
We present central CaT, PaT, and CaT* indices for a sample of fifteen dwarf
elliptical galaxies (dEs). Twelve of these have CaT* ~ 7 A and extend the
negative correlation between the CaT* index and central velocity dispersion
sigma, which was derived for bright ellipticals (Es), down to 20 < sigma < 55
km/s. For five dEs we have independent age and metallicity estimates. Four of
these have CaT* ~ 7 A, much higher than expected from their low metallicities
(-1.5 < [Z/H] < -0.5). The observed anti-correlation of CaT* as a function of
sigma or Z is in flagrant disagreement with theory. We discuss some of the
amendments that have been proposed to bring the theoretical predictions into
agreement with the observed CaT*-values of bright Es and how they can be
extended to incorporate also the observed CaT*-values of dEs. Moreover, 3 dEs
in our sample have CaT* ~ 5 A, as would be expected for metal-poor stellar
systems. Any theory for dE evolution will have to be able to explain the
co-existence of low-CaT* and high-CaT* dEs at a given mean metallicity. This
could be the first direct evidence that the dE population is not homogeneous,
and that different evolutionary paths led to morphologically and kinematically
similar but chemically distinct objects.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Full characterization of a three-photon GHZ state using quantum state tomography
We have performed the first experimental tomographic reconstruction of a
three-photon polarization state. Quantum state tomography is a powerful tool
for fully describing the density matrix of a quantum system. We measured 64
three-photon polarization correlations and used a "maximum-likelihood"
reconstruction method to reconstruct the GHZ state. The entanglement class has
been characterized using an entanglement witness operator and the maximum
predicted values for the Mermin inequality was extracted.Comment: 3 pages, 3 figure
Comment on "Exclusion of time in the theorem of Bell" by K. Hess and W. Philipp
A recent Letter by Hess and Philipp claims that Bell's theorem neglects the
possibility of time-like dependence in local hidden variables, hence is not
conclusive. Moreover the authors claim that they have constructed, in an
earlier paper, a local realistic model of the EPR correlations. However, they
themselves have neglected the experimenter's freedom to choose settings, while
on the other hand, Bell's theorem can be formulated to cope with time-like
dependence. This in itself proves that their toy model cannot satisfy local
realism, but we also indicate where their proof of its local realistic nature
fails.Comment: Latex needs epl.cl
A high-reflectivity high-Q micromechanical Bragg-mirror
We report on the fabrication and characterization of a micromechanical
oscillator consisting only of a free-standing dielectric Bragg mirror with high
optical reflectivity and high mechanical quality. The fabrication technique is
a hybrid approach involving laser ablation and dry etching. The mirror has a
reflectivity of 99.6%, a mass of 400ng, and a mechanical quality factor Q of
approximately 10^4. Using this micromirror in a Fabry Perot cavity, a finesse
of 500 has been achieved. This is an important step towards designing tunable
high-Q high-finesse cavities on chip.Comment: 3 pages, 2 figure
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