801 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
Representation of SO(3) Group by a Maximally Entangled State
A representation of the SO(3) group is mapped into a maximally entangled two
qubit state according to literatures. To show the evolution of the entangled
state, a model is set up on an maximally entangled electron pair, two electrons
of which pass independently through a rotating magnetic field. It is found that
the evolution path of the entangled state in the SO(3) sphere breaks an odd or
even number of times, corresponding to the double connectedness of the SO(3)
group. An odd number of breaks leads to an additional phase to the
entangled state, but an even number of breaks does not. A scheme to trace the
evolution of the entangled state is proposed by means of entangled photon pairs
and Kerr medium, allowing observation of the additional phase.Comment: 4 pages, 3 figure
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
Matter-wave interferometer for large molecules
We demonstrate a near-field Talbot-Lau interferometer for C-70 fullerene
molecules. Such interferometers are particularly suitable for larger masses.
Using three free-standing gold gratings of one micrometer period and a
transversally incoherent but velocity-selected molecular beam, we achieve an
interference fringe visibility of 40 % with high count rate. Both the high
visibility and its velocity dependence are in good agreement with a quantum
simulation that takes into account the van der Waals interaction of the
molecules with the gratings and are in striking contrast to a classical moire
model.Comment: revtex4, 4 pages, 3 figure
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
High-fidelity entanglement swapping with fully independent sources
Entanglement swapping allows to establish entanglement between independent
particles that never interacted nor share any common past. This feature makes
it an integral constituent of quantum repeaters. Here, we demonstrate
entanglement swapping with time-synchronized independent sources with a
fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more
than four standard deviations. The fact that both entangled pairs are created
by fully independent, only electronically connected sources ensures that this
technique is suitable for future long-distance quantum communication
experiments as well as for novel tests on the foundations of quantum physics.Comment: added technical details and extended introduction and conclusion,
slightly modified the abstract, corrected a mistake in the affiliation
A posteriori teleportation
The article by Bouwmeester et al. on experimental quantum teleportation
constitutes an important advance in the burgeoning field of quantum
information. The experiment was motivated by the proposal of Bennett et al. in
which an unknown quantum state is `teleported' by Alice to Bob. As illustrated
in Fig. 1, in the implementation of this procedure, by Bouwmeester et al., an
input quantum state is `disembodied' into quantum and classical components, as
in the original protocol. However, in contrast to the original scheme,
Bouwmeester et al.'s procedure necessarily destroys the state at Bob's
receiving terminal, so a `teleported' state can never emerge as a freely
propagating state for subsequent examination or exploitation. In fact,
teleportation is achieved only as a postdiction.Comment: 1 page LaTeX including 1 figure. Scientific Correspondence about:
"Experimental quantum teleportation" Nature 390, 575 (1997
Use of entanglement in quantum optics
Several recent demonstrations of two-particle interferometry are reviewed and shown to be examples of either color entanglement or beam entanglement. A device, called a number filter, is described and shown to be of value in preparing beam entanglements. Finally, we note that all three concepts (color and beam entaglement, and number filtering) may be extended to three or more particles
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
Divergence of an orbital-angular-momentum-carrying beam upon propagation
There is recent interest in the use of light beams carrying orbital angular
momentum (OAM) for creating multiple channels within free-space optical
communication systems. One limiting issue is that, for a given beam size at the
transmitter, the beam divergence angle increases with increasing OAM, thus
requiring a larger aperture at the receiving optical system if the efficiency
of detection is to be maintained. Confusion exists as to whether this
divergence scales linarly with, or with the square root of, the beam's OAM. We
clarify how both these scaling laws are valid, depending upon whether it is the
radius of the Gaussian beam waist or the rms intensity which is kept constant
while varying the OAM.Comment: 4 pages, 2 figure
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