30,050 research outputs found
Quantitative conditional quantum erasure in two-atom resonance fluorescence
We present a conditional quantum eraser which erases the a priori knowledge
or the predictability of the path a photon takes in a Young-type double-slit
experiment with two fluorescent four-level atoms. This erasure violates a
recently derived erasure relation which must be satisfied for a conventional,
unconditional quantum eraser that aims to find an optimal sorting of the system
into subensembles with particularly large fringe visibilities. The conditional
quantum eraser employs an interaction-free, partial which-way measurement which
not only sorts the system into optimal subsystems with large visibility but
also selects the appropriate subsystem with the maximum possible visibility. We
explain how the erasure relation can be violated under these circumstances.Comment: Revtex4, 12pages, 4 eps figures, replaced with published version,
changes in Sec. 3, to appear in Physical Review
Effect of frequency mismatched photons in quantum information processing
Many promising schemes for quantum information processing (QIP) rely on
few-photon interference effects. In these proposals, the photons are treated as
being indistinguishable particles. However, single photon sources are typically
subject to variation from device to device. Thus the photons emitted from
different sources will not be perfectly identical, and there will be some
variation in their frequencies. Here, we analyse the effect of this frequency
mismatch on QIP schemes. As examples, we consider the distributed QIP protocol
proposed by Barrett and Kok, and Hong-Ou-Mandel interference which lies at the
heart of many linear optical schemes for quantum computing. In the distributed
QIP protocol, we find that the fidelity of entangled qubit states depends
crucially on the time resolution of single photon detectors. In particular,
there is no reduction in the fidelity when an ideal detector model is assumed,
while reduced fidelities may be encountered when using realistic detectors with
a finite response time. We obtain similar results in the case of Hong-Ou-Mandel
interference -- with perfect detectors, a modified version of quantum
interference is seen, and the visibility of the interference pattern is reduced
as the detector time resolution is reduced. Our findings indicate that problems
due to frequency mismatch can be overcome, provided sufficiently fast detectors
are available.Comment: 14 pages, 8 figures. Comments welcome. v2: Minor changes. v3: Cleaned
up 3 formatting error
Optical tests of Bell's inequalities not resting upon the absurd fair sampling assumption
A simple local hidden-variables model is exhibited which reproduces the
results of all performed tests of Bell\'{}s inequalities involving optical
photon pairs. For the old atomic-cascade experiments, like Aspect\'{}s, the
model agrees with quantum mechanics even for ideal set-ups. For more recent
experiments, using parametric down-converted photons, the agreement occurs only
for actual experiments, involving low efficiency detectors. Arguments are given
against the fair sampling assumption, currently combined with the results of
the experiments in order to claim a contradiction with local realism. New tests
are proposed which are able to discriminate between quantum mechanics and a
restricted, but appealing, family of local hidden-variables models. Such tests
require detectors with efficiencies just above 20%.Comment: 19 page
Formal proofs for broadcast algorithms
Standard distributed algorithmic solutions to recurring distributed problems are commonly specified and described informally. A proper understanding of these distributed algorithms that clarifies ambiguities requires formal descriptions. However, formalisation tends to yield complex descriptions. We formally study two broadcast algorithms and present an encoding framework using a process descriptive language and formalise these algorithms and their specifications using this framework. Following these new formal encodings we discuss correctness proofs for the same algorithms.peer-reviewe
States in Process Calculi
Formal reasoning about distributed algorithms (like Consensus) typically
requires to analyze global states in a traditional state-based style. This is
in contrast to the traditional action-based reasoning of process calculi.
Nevertheless, we use domain-specific variants of the latter, as they are
convenient modeling languages in which the local code of processes can be
programmed explicitly, with the local state information usually managed via
parameter lists of process constants. However, domain-specific process calculi
are often equipped with (unlabeled) reduction semantics, building upon a rich
and convenient notion of structural congruence. Unfortunately, the price for
this convenience is that the analysis is cumbersome: the set of reachable
states is modulo structural congruence, and the processes' state information is
very hard to identify. We extract from congruence classes of reachable states
individual state-informative representatives that we supply with a proper
formal semantics. As a result, we can now freely switch between the process
calculus terms and their representatives, and we can use the stateful
representatives to perform assertional reasoning on process calculus models.Comment: In Proceedings EXPRESS/SOS 2014, arXiv:1408.127
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