403 research outputs found
On the solution of trivalent decision problems by quantum state identification
Full text linkThe trivalent functions of a trit can be grouped into equipartitions of three
elements. We discuss the separation of the corresponding functional classes by
quantum state identifications
Bell's experiment with intra- and inter-pair entanglement: Single-particle mode entanglement as a case study
Full text linkTheoretical considerations of Bell-inequality experiments usually assume
identically prepared and independent pairs of particles. Here we consider pairs
that exhibit both intra- and inter-pair entanglement. The pairs are taken from
a large many-body system where all the pairs are generally entangled with each
other. Using an explicit example based on single mode entanglement and an
ancillary Bose-Einstein condensate, we show that the Bell-inequality violation
in such systems can display statistical properties that are remarkably
different from those obtained using identically prepared, independent pairs. In
particular, one can have probabilistic violation of Bell's inequalities in
which a finite fraction of all the runs result in violation, even though there
could be no violation when averaging over all the runs. Whether or not a
particular run of results will end up being local realistically explainable is
"decided" by a sequence of quantum (random) outcomes.Comment: 7 pages (two column), 5 figure
Information and The Brukner-Zeilinger Interpretation of Quantum Mechanics: A Critical Investigation
Full text linkIn Brukner and Zeilinger's interpretation of quantum mechanics, information
is introduced as the most fundamental notion and the finiteness of information
is considered as an essential feature of quantum systems. They also define a
new measure of information which is inherently different from the Shannon
information and try to show that the latter is not useful in defining the
information content in a quantum object.
Here, we show that there are serious problems in their approach which make
their efforts unsatisfactory. The finiteness of information does not explain
how objective results appear in experiments and what an instantaneous change in
the so-called information vector (or catalog of knowledge) really means during
the measurement. On the other hand, Brukner and Zeilinger's definition of a new
measure of information may lose its significance, when the spin measurement of
an elementary system is treated realistically. Hence, the sum of the individual
measures of information may not be a conserved value in real experiments.Comment: 20 pages, two figures, last version. Section 4 is replaced by a new
argument. Other sections are improved. An appendix and new references are
adde
Classical world arising out of quantum physics under the restriction of coarse-grained measurements
Full text linkConceptually different from the decoherence program, we present a novel
theoretical approach to macroscopic realism and classical physics within
quantum theory. It focuses on the limits of observability of quantum effects of
macroscopic objects, i.e., on the required precision of our measurement
apparatuses such that quantum phenomena can still be observed. First, we
demonstrate that for unrestricted measurement accuracy no classical description
is possible for arbitrarily large systems. Then we show for a certain time
evolution that under coarse-grained measurements not only macrorealism but even
the classical Newtonian laws emerge out of the Schroedinger equation and the
projection postulate.Comment: 4 pages, 1 figure, second revised and published versio
The conditions for quantum violation of macroscopic realism
Full text linkWhy do we not experience a violation of macroscopic realism in every-day
life? Normally, no violation can be seen either because of decoherence or the
restriction of coarse-grained measurements, transforming the time evolution of
any quantum state into a classical time evolution of a statistical mixture. We
find the sufficient condition for these classical evolutions for spin systems
under coarse-grained measurements. Then we demonstrate that there exist
"non-classical" Hamiltonians whose time evolution cannot be understood
classically, although at every instant of time the quantum spin state appears
as a classical mixture. We suggest that such Hamiltonians are unlikely to be
realized in nature because of their high computational complexity.Comment: 4 pages, 2 figures, revised version, journal reference adde
Logical independence and quantum randomness
Full text linkWe propose a link between logical independence and quantum physics. We
demonstrate that quantum systems in the eigenstates of Pauli group operators
are capable of encoding mathematical axioms and show that Pauli group quantum
measurements are capable of revealing whether or not a given proposition is
logically dependent on the axiomatic system. Whenever a mathematical
proposition is logically independent of the axioms encoded in the measured
state, the measurement associated with the proposition gives random outcomes.
This allows for an experimental test of logical independence. Conversely, it
also allows for an explanation of the probabilities of random outcomes observed
in Pauli group measurements from logical independence without invoking quantum
theory. The axiomatic systems we study can be completed and are therefore not
subject to Goedel's incompleteness theorem.Comment: 9 pages, 4 figures, published version plus additional experimental
appendi
Crucial Role of Quantum Entanglement in Bulk Properties of Solids
Full text linkWe demonstrate that the magnetic susceptibility of strongly alternating
antiferromagnetic spin-1/2 chains is an entanglement witness. Specifically,
magnetic susceptibility of copper nitrate (CN) measured in 1963 (Berger et al.,
Phys. Rev. 132, 1057 (1963)) cannot be described without presence of
entanglement. A detailed analysis of the spin correlations in CN as obtained
from neutron scattering experiments (Xu et al., Phys. Rev. Lett. 84, 4465
(2000)) provides microscopic support for this interpretation. We present a
quantitative analysis resulting in the critical temperature of 5K in both,
completely independent, experiments below which entanglement exists.Comment: 4 pages, 2 figure
Quantum nonlocality obtained from local states by entanglement purification
Full text linkWe have applied an entanglement purification protocol to produce a single
entangled pair of photons capable of violating a CHSH Bell inequality from two
pairs that individually could not. The initial poorly-entangled photons were
created by a controllable decoherence that introduced complex errors. All of
the states were reconstructed using quantum state tomography which allowed for
a quantitative description of the improvement of the state after purification.Comment: 4 pages, 4 figure
A scheme for entanglement extraction from a solid
Full text linkSome thermodynamical properties of solids, such as heat capacity and magnetic
susceptibility, have recently been shown to be linked to the amount of
entanglement in a solid. However this entanglement may appear a mere
mathematical artifact of the typical symmetrization procedure of many-body wave
function in solid state physics.
Here we show that this entanglement is physical demonstrating the principles
of its extraction from a typical solid state system by scattering two particles
off the system. Moreover we show how to simulate this process using present-day
optical lattices technology. This demonstrates not only that entanglement
exists in solids but also that it can be used for quantum information
processing or for test of Bell's inequalities.Comment: 10 pages, 3 figures, published versio
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