32,344 research outputs found
Experimenter's Freedom in Bell's Theorem and Quantum Cryptography
Bell's theorem states that no local realistic explanation of quantum
mechanical predictions is possible, in which the experimenter has a freedom to
choose between different measurement settings. Within a local realistic picture
the violation of Bell's inequalities can only be understood if this freedom is
denied. We determine the minimal degree to which the experimenter's freedom has
to be abandoned, if one wants to keep such a picture and be in agreement with
the experiment. Furthermore, the freedom in choosing experimental arrangements
may be considered as a resource, since its lacking can be used by an
eavesdropper to harm the security of quantum communication. We analyze the
security of quantum key distribution as a function of the (partial) knowledge
the eavesdropper has about the future choices of measurement settings which are
made by the authorized parties (e.g. on the basis of some quasi-random
generator). We show that the equivalence between the violation of Bell's
inequality and the efficient extraction of a secure key - which exists for the
case of complete freedom (no setting knowledge) - is lost unless one adapts the
bound of the inequality according to this lack of freedom.Comment: 7 pages, 2 figures, incorporated referee comment
Bilocal versus non-bilocal correlations in entanglement swapping experiments
Entanglement swapping is a process by which two initially independent quantum
systems can become entangled and generate nonlocal correlations. To
characterize such correlations, we compare them to those predicted by bilocal
models, where systems that are initially independent are described by
uncorrelated states. We extend in this paper the analysis of bilocal
correlations initiated in [Phys. Rev. Lett. 104, 170401 (2010)]. In particular,
we derive new Bell-type inequalities based on the bilocality assumption in
different scenarios, we study their possible quantum violations, and analyze
their resistance to experimental imperfections. The bilocality assumption,
being stronger than Bell's standard local causality assumption, lowers the
requirements for the demonstration of quantumness in entanglement swapping
experiments
Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate \textit{Kepler} rotation periods
The study of stellar activity cycles is crucial to understand the underlying
dynamo and how it causes activity signatures such as dark spots and bright
faculae. We study the appearance of activity signatures in contemporaneous
photometric and chromospheric time series. Lomb-Scargle periodograms are used
to search for cycle periods present in both time series. To emphasize the
signature of the activity cycle we account for rotation-induced scatter in both
data sets by fitting a quasi-periodic Gaussian process model to each observing
season. After subtracting the rotational variability, cycle amplitudes and the
phase difference between the two time series are obtained by fitting both time
series simultaneously using the same cycle period. We find cycle periods in 27
of the 30 stars in our sample. The phase difference between the two time series
reveals that the variability in fast rotating active stars is usually in
anti-phase, while the variability of slowly rotating inactive stars is in
phase. The photometric cycle amplitudes are on average six times larger for the
active stars. The phase and amplitude information demonstrates that active
stars are dominated by dark spots, whereas less active stars are dominated by
bright faculae. We find the transition from spot to faculae domination at the
Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that
faculae are the dominant ingredient of stellar activity cycles at ages >2.55
Gyr. The data further suggest that the Vaughan-Preston gap can not explain the
previously detected dearth of Kepler rotation periods between 15-25 days.
Nevertheless, our results led us to propose an explanation for the rotation
period dearth to be due to the non-detection of periodicity caused by the
cancellation of dark spots and bright faculae at 800 Myr.Comment: 12+15 pages, 10+2 figures, accepted for publication in A&
Emergence of Quantum Correlations from Non-Locality Swapping
By studying generalized non-signalling theories, the hope is to find out what
makes quantum mechanics so special. In the present paper, we revisit the
paradigmatic model of non-signalling boxes and introduce the concept of a
genuine box. This will allow us to present the first generalized non-signalling
model featuring quantum-like dynamics. In particular, we present the coupler, a
device enabling non-locality swapping, the analogue of quantum entanglement
swapping, as well as teleportation. Remarkably, part of the boundary between
quantum and post-quantum correlations emerges in our study.Comment: 5 pages. 6 figures. Minor Revisions. To appear in PR
The Origin of Episodic Accretion Bursts in the Early Stages of Star Formation
We study numerically the evolution of rotating cloud cores, from the collapse
of a magnetically supercritical core to the formation of a protostar and the
development of a protostellar disk during the main accretion phase. We find
that the disk quickly becomes unstable to the development of a spiral structure
similar to that observed recently in AB Aurigae. A continuous infall of matter
from the protostellar envelope makes the protostellar disk unstable, leading to
spiral arms and the formation of dense protostellar/protoplanetary clumps
within them. The growing strength of spiral arms and ensuing redistribution of
mass and angular momentum creates a strong centrifugal disbalance in the disk
and triggers bursts of mass accretion during which the dense
protostellar/protoplanetary clumps fall onto the central protostar. These
episodes of clump infall may manifest themselves as episodes of vigorous
accretion rate (\ge 10^{-4} M_sun/yr) as is observed in FU Orionis variables.
Between these accretion bursts, the protostar is characterized by a low
accretion rate (< 10^{-6} M_sun/yr). During the phase of episodic accretion,
the mass of the protostellar disk remains less than or comparable to the mass
of the protostar.Comment: 5 pages, 2 figures, accepted for publication in ApJ
Long-range quantum entanglement in noisy cluster states
We describe a phase transition for long-range entanglement in a
three-dimensional cluster state affected by noise. The partially decohered
state is modeled by the thermal state of a suitable Hamiltonian. We find that
the temperature at which the entanglement length changes from infinite to
finite is nonzero. We give an upper and lower bound to this transition
temperature.Comment: 7 page
J.S. Bell's Concept of Local Causality
John Stewart Bell's famous 1964 theorem is widely regarded as one of the most
important developments in the foundations of physics. It has even been
described as "the most profound discovery of science." Yet even as we approach
the 50th anniversary of Bell's discovery, its meaning and implications remain
controversial. Many textbooks and commentators report that Bell's theorem
refutes the possibility (suggested especially by Einstein, Podolsky, and Rosen
in 1935) of supplementing ordinary quantum theory with additional ("hidden")
variables that might restore determinism and/or some notion of an
observer-independent reality. On this view, Bell's theorem supports the
orthodox Copenhagen interpretation. Bell's own view of his theorem, however,
was quite different. He instead took the theorem as establishing an "essential
conflict" between the now well-tested empirical predictions of quantum theory
and relativistic \emph{local causality}. The goal of the present paper is, in
general, to make Bell's own views more widely known and, in particular, to
explain in detail Bell's little-known mathematical formulation of the concept
of relativistic local causality on which his theorem rests. We thus collect and
organize many of Bell's crucial statements on these topics, which are scattered
throughout his writings, into a self-contained, pedagogical discussion
including elaborations of the concepts "beable", "completeness", and
"causality" which figure in the formulation. We also show how local causality
(as formulated by Bell) can be used to derive an empirically testable Bell-type
inequality, and how it can be used to recapitulate the EPR argument.Comment: 19 pages, 4 figure
Quantum Correlation Bounds for Quantum Information Experiments Optimization: the Wigner Inequality Case
Violation of modified Wigner inequality by means binary bipartite quantum
system allows the discrimination between the quantum world and the classical
local-realistic one, and also ensures the security of Ekert-like quantum key
distribution protocol. In this paper we study both theoretically and
experimentally the bounds of quantum correlation associated to the modified
Wigner's inequality finding the optimal experimental configuration for its
maximal violation. We also extend this analysis to the implementation of
Ekert's protocol
The orbital poles of Milky Way satellite galaxies: a rotationally supported disc-of-satellites
Available proper motion measurements of Milky Way (MW) satellite galaxies are
used to calculate their orbital poles and projected uncertainties. These are
compared to a set of recent cold dark-matter (CDM) simulations, tailored
specifically to solve the MW satellite problem. We show that the CDM satellite
orbital poles are fully consistent with being drawn from a random distribution,
while the MW satellite orbital poles indicate that the disc-of-satellites of
the Milky Way is rotationally supported. Furthermore, the bootstrapping
analysis of the spatial distribution of theoretical CDM satellites also shows
that they are consistent with being randomly drawn. The theoretical CDM
satellite population thus shows a significantly different orbital and spatial
distribution than the MW satellites, most probably indicating that the majority
of the latter are of tidal origin rather than being DM dominated
sub-structures. A statistic is presented that can be used to test a possible
correlation of satellite galaxy orbits with their spatial distribution.Comment: Accepted for publication in Ap
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