165 research outputs found
Necessary and sufficient conditions for macroscopic realism from quantum mechanics
Macroscopic realism, the classical world view that macroscopic objects exist
independently of and are not influenced by measurements, is usually tested
using Leggett-Garg inequalities. Recently, another necessary condition called
no-signaling in time (NSIT) has been proposed as a witness for non-classical
behavior. In this paper, we show that a combination of NSIT conditions is not
only necessary but also sufficient for a macrorealistic description of a
physical system. Any violation of macroscopic realism must therefore be
witnessed by a suitable NSIT condition. Subsequently, we derive an operational
formulation for NSIT in terms of positive operator-valued measurements and the
system Hamiltonian. We argue that this leads to a suitable definition of
"classical" measurements and Hamiltonians, and apply our formalism to some
generic coarse-grained quantum measurements.Comment: 10 pages, published versio
Condition for macroscopic realism beyond the Leggett-Garg inequalities
In 1985, Leggett and Garg put forward the concept of macroscopic realism
(macrorealism) and, in analogy to Bell's theorem, derived a necessary condition
in terms of inequalities, which are now known as the Leggett-Garg inequalities.
In this paper, we discuss another necessary condition called no-signaling in
time. It solely bases on comparing the probability distribution for a
macrovariable at some time for the cases where previously a measurement has or
has not been performed. Although the concept is analogous to the no-signaling
condition in the case of Bell tests, it can be violated according to quantum
mechanical predictions even in situations where no violation of Leggett-Garg
inequalities is possible.Comment: 6 pages, 1 table, 1 figure, published versio
Entanglement distribution revealed by macroscopic observations
What can we learn about entanglement between individual particles in
macroscopic samples by observing only the collective properties of the
ensembles? Using only a few experimentally feasible collective properties, we
establish an entanglement measure between two samples of spin-1/2 particles (as
representatives of two-dimensional quantum systems). This is a tight lower
bound for the average entanglement between all pairs of spins in general and is
equal to the average entanglement for a certain class of systems. We compute
the entanglement measures for explicit examples and show how to generalize the
method to more than two samples and multi-partite entanglement.Comment: 4 pages, 2 figures, revised and published versio
A Snapshot of Foundational Attitudes Toward Quantum Mechanics
Foundational investigations in quantum mechanics, both experimental and
theoretical, gave birth to the field of quantum information science.
Nevertheless, the foundations of quantum mechanics themselves remain hotly
debated in the scientific community, and no consensus on essential questions
has been reached. Here, we present the results of a poll carried out among 33
participants of a conference on the foundations of quantum mechanics. The
participants completed a questionnaire containing 16 multiple-choice questions
probing opinions on quantum-foundational issues. Participants included
physicists, philosophers, and mathematicians. We describe our findings,
identify commonly held views, and determine strong, medium, and weak
correlations between the answers. Our study provides a unique snapshot of
current views in the field of quantum foundations, as well as an analysis of
the relationships between these views.Comment: 17 pages, 3 figure
Light polarization measurements in tests of macrorealism
According to the world view of macrorealism, the properties of a given system
exist prior to and independent of measurement, which is incompatible with
quantum mechanics. Leggett and Garg put forward a practical criterion capable
of identifying violations of macrorealism, and so far experiments performed on
microscopic and mesoscopic systems have always ruled out in favor of quantum
mechanics. However, a macrorealist can always assign the cause of such
violations to the perturbation that measurements effect on such small systems,
and hence a definitive test would require using non-invasive measurements,
preferably on macroscopic objects, where such measurements seem more plausible.
However, the generation of truly macroscopic quantum superposition states
capable of violating macrorealism remains a big challenge. In this work we
propose a setup that makes use of measurements on the polarization of light, a
property which has been extensively manipulated both in classical and quantum
contexts, hence establishing the perfect link between the microscopic and
macroscopic worlds. In particular, we use Leggett-Garg inequalities and the
criterion of no-signaling in time to study the macrorealistic character of
light polarization for different kinds of measurements, in particular with
different degrees of coarse-graining. Our proposal is non-invasive for coherent
input states by construction. We show for states with well defined photon
number in two orthogonal polarization modes, that there always exists a way of
making the measurement sufficiently coarse-grained so that a violation of
macrorealism becomes arbitrarily small, while sufficiently sharp measurements
can always lead to a significant violation.Comment: Comments, suggestions, and constructive criticism are welcom
Delayed-choice gedanken experiments and their realizations
The wave-particle duality dates back to Einstein's explanation of the
photoelectric effect through quanta of light and de Broglie's hypothesis of
matter waves. Quantum mechanics uses an abstract description for the behavior
of physical systems such as photons, electrons, or atoms. Whether quantum
predictions for single systems in an interferometric experiment allow an
intuitive understanding in terms of the particle or wave picture, depends on
the specific configuration which is being used. In principle, this leaves open
the possibility that quantum systems always either behave definitely as a
particle or definitely as a wave in every experimental run by a priori adapting
to the specific experimental situation. This is precisely what is tried to be
excluded by delayed-choice experiments, in which the observer chooses to reveal
the particle or wave character -- or even a continuous transformation between
the two -- of a quantum system at a late stage of the experiment. We review the
history of delayed-choice gedanken experiments, which can be traced back to the
early days of quantum mechanics. Then we discuss their experimental
realizations, in particular Wheeler's delayed choice in interferometric setups
as well as delayed-choice quantum erasure and entanglement swapping. The latter
is particularly interesting, because it elevates the wave-particle duality of a
single quantum system to an entanglement-separability duality of multiple
systems
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