9,186 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

### 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.

### Infinite matrices may violate the associative law

The momentum operator for a particle in a box is represented by an infinite
order Hermitian matrix $P$. Its square $P^2$ is well defined (and diagonal),
but its cube $P^3$ is ill defined, because $P P^2\neq P^2 P$. Truncating these
matrices to a finite order restores the associative law, but leads to other
curious results.Comment: final version in J. Phys. A28 (1995) 1765-177

### Understanding Popper's Experiment

An experiment proposed by Karl Popper is considered by many to be a crucial
test of quantum mechanics. Although many loopholes in the original proposal
have been pointed out, they are not crucial to the test. We use only the
standard interpretation of quantum mechanics to point out what is fundamentally
wrong with the proposal, and demonstrate that Popper's basic premise was
faulty.Comment: Edited version, to appear in Am. J. Phy

### The Effects of Symmetries on Quantum Fidelity Decay

We explore the effect of a system's symmetries on fidelity decay behavior.
Chaos-like exponential fidelity decay behavior occurs in non-chaotic systems
when the system possesses symmetries and the applied perturbation is not tied
to a classical parameter. Similar systems without symmetries exhibit
faster-than-exponential decay under the same type of perturbation. This
counter-intuitive result, that extra symmetries cause the system to behave in a
chaotic fashion, may have important ramifications for quantum error correction.Comment: 5 pages, 3 figures, to be published Phys. Rev. E Rapid Communicatio

### Bell's inequality with Dirac particles

We study Bell's inequality using the Bell states constructed from four
component Dirac spinors. Spin operator is related to the Pauli-Lubanski pseudo
vector which is relativistic invariant operator. By using Lorentz
transformation, in both Bell states and spin operator, we obtain an observer
independent Bell's inequality, so that it is maximally violated as long as it
is violated maximally in the rest frame.Comment: 7 pages. arXiv admin note: text overlap with arXiv:quant-ph/0308156
by other author

### Optimal distinction between non-orthogonal quantum states

Given a finite set of linearly independent quantum states, an observer who
examines a single quantum system may sometimes identify its state with
certainty. However, unless these quantum states are orthogonal, there is a
finite probability of failure. A complete solution is given to the problem of
optimal distinction of three states, having arbitrary prior probabilities and
arbitrary detection values. A generalization to more than three states is
outlined.Comment: 9 pages LaTeX, one PostScript figure on separate pag

### Quantum mechanics explained

The physical motivation for the mathematical formalism of quantum mechanics
is made clear and compelling by starting from an obvious fact - essentially,
the stability of matter - and inquiring into its preconditions: what does it
take to make this fact possible?Comment: 29 pages, 5 figures. v2: revised in response to referee comment

### Maximally-Disordered Distillable Quantum States

We explore classical to quantum transition of correlations by studying the
quantum states located just outside of the classically-correlated-states-only
neighborhood of the maximally mixed state (the largest separable ball (LSB)).
We show that a natural candidate for such states raises the possibility of a
layered transition, i.e., an annular region comprising only classical and the
classical-like bound entangled states, followed by free or distillable
entanglement. Surprisingly, we find the transition to be abrupt for bipartite
systems: distillable states emerge arbitrarily close to the LSB. For
multipartite systems, while the radius of the LSB remains unknown, we determine
the radius of the largest undistillable ball. Our results also provide an upper
bound on how noisy shared entangled states can be for executing quantum
information processing protocols.Comment: Published Version, 7 pages, Late

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