31,750 research outputs found
Delayed choice for entanglement swapping
Two observers (Alice and Bob) independently prepare two sets of singlets.
They test one particle of each singlet along an arbitrarily chosen direction
and send the other particle to a third observer, Eve. At a later time, Eve
performs joint tests on pairs of particles (one from Alice and one from Bob).
According to Eve's choice of test and to her results, Alice and Bob can sort
into subsets the samples that they have already tested, and they can verify
that each subset behaves as if it consisted of entangled pairs of distant
particles, that have never communicated in the past, even indirectly via other
particles.Comment: 7 pages, LaTeX, to appear in special issue of J. Modern Optic
Reply to the comment of Y. Aharonov and L. Vaidman on ``Time asymmetry in quantum mechanics: a retrodiction paradox''
In the standard physical interpretation of quantum theory, prediction and
retrodiction are not symmetric. The opposite assertion by some authors results
from their use of non-standard interpretations of the theory.Comment: final version in Physics Letters A 203 (1995) 15
Depurification by Lorentz boosts
We consider a particle of half-integer spin which is nonrelativistic in the
rest frame. Assuming the particle is completely polarized along third axis we
calculate the Bloch vector as seen by a moving observer. The result for its
length is expressed in terms of dispersion of some vector operator linear in
momentum. The relation with the localization properties is discussed.Comment: 5 page
Strategies to measure a quantum state
We consider the problem of determining the mixed quantum state of a large but
finite number of identically prepared quantum systems from data obtained in a
sequence of ideal (von Neumann) measurements, each performed on an individual
copy of the system. In contrast to previous approaches, we do not average over
the possible unknown states but work out a ``typical'' probability distribution
on the set of states, as implied by the experimental data. As a consequence,
any measure of knowledge about the unknown state and thus any notion of ``best
strategy'' (i.e. the choice of observables to be measured, and the number of
times they are measured) depend on the unknown state. By learning from
previously obtained data, the experimentalist re-adjusts the observable to be
measured in the next step, eventually approaching an optimal strategy. We
consider two measures of knowledge and exhibit all ``best'' strategies for the
case of a two-dimensional Hilbert space. Finally, we discuss some features of
the problem in higher dimensions and in the infinite dimensional case.Comment: 32 pages, Late
What's Wrong with these Observables?
An imprecise measurement of a dynamical variable (such as a spin component)
does not, in general, give the value of another dynamical variable (such as a
spin component along a slightly different direction). The result of the
measurement cannot be interpreted as the value of any observable that has a
classical analogue.Comment: submitted to Foundations of Physic
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