18 research outputs found
Measurement dependent locality
The demonstration and use of Bell-nonlocality, a concept that is
fundamentally striking and is at the core of applications in device independent
quantum information processing, relies heavily on the assumption of measurement
independence, also called the assumption of free choice. The latter cannot be
verified or guaranteed. In this paper, we consider a relaxation of the
measurement independence assumption. We briefly review the results of Phys.
Rev. Lett. 113, 190402 (2014), which show that with our relaxation, the set of
so-called measurement dependent local (MDL) correlations is a polytope, i.e. it
can be fully described using a finite set of linear inequalities. Here we
analyze this polytope, first in the simplest case of 2 parties with binary
inputs and outputs, for which we give a full characterization. We show that
partially entangled states are preferable to the maximally entangled state when
dealing with measurement dependence in this scenario. We further present a
method which transforms any Bell-inequality into an MDL inequality and give
valid inequalities for the case of arbitrary number of parties as well as one
for arbitrary number of inputs. We introduce the assumption of independent
sources in the measurement dependence scenario and give a full analysis for the
bipartite scenario with binary inputs and outputs. Finally, we establish a link
between measurement dependence and another strong hindrance in certifying
nonlocal correlations: nondetection events.Comment: 16+7 pages, 2 figure
Demonstration of Quantum Nonlocality in presence of Measurement Dependence
Quantum nonlocality stands as a resource for Device Independent Quantum
Information Processing (DIQIP), as, for instance, Device Independent Quantum
Key Distribution. We investigate experimentally the assumption of limited
Measurement Dependence, i.e., that the measurement settings used in Bell
inequality tests or DIQIP are partially influenced by the source of entangled
particle and/or by an adversary. Using a recently derived Bell-like inequality
[Phys. Rev. Lett. 113 190402] and a 99% fidelity source of partially entangled
polarization photonic qubits, we obtain a clear violation of the inequality,
excluding a much larger range of measurement dependent local models than would
be possible with an adapted Clauser, Horne, Shimony and Holt (CHSH) inequality.
It is therefore shown that the Measurement Independence assumption can be
widely relaxed while still demonstrating quantum nonlocality
Nonlinear Bell inequalities tailored for quantum networks
In a quantum network, distant observers sharing physical resources emitted by
independent sources can establish strong correlations, which defy any classical
explanation in terms of local variables. We discuss the characterization of
nonlocal correlations in such a situation, when compared to those that can be
generated in networks distributing independent local variables. We present an
iterative procedure for constructing Bell inequalities tailored for networks:
starting from a given network, and a corresponding Bell inequality, our
technique provides new Bell inequalities for a more complex network, involving
one additional source and one additional observer. The relevance of our method
is illustrated on a variety of networks, for which we demonstrate significant
quantum violations.Comment: 8 pages, 2 figures. Comments welcom
Quantum clocks are more accurate than classical ones
A clock is, from an information-theoretic perspective, a system that emits
information about time. One may therefore ask whether the theory of information
imposes any constraints on the maximum precision of clocks. Here we show a
quantum-over-classical advantage for clocks or, more precisely, the task of
generating information about what time it is. The argument is based on
information-theoretic considerations: we analyse how the accuracy of a clock
scales with its size, measured in terms of the number of bits that could be
stored in it. We find that a quantum clock can achieve a quadratically improved
accuracy compared to a purely classical one of the same size.Comment: 17 + 60 pages. V2: corrected typos and improved discussio
Family of bell-like inequalities as device-independent witnesses for entanglement depth
Physical Review Letters1141919040
Nonlocality of and Dicke states subject to losses
We discuss the nonlocality of the and the Dicke states subject to losses.
We consider two noise models, namely loss of excitations and loss of particles,
and investigate how much loss can be tolerated such that the final state
remains nonlocal. This leads to a measure of robustness of the nonlocality of
Dicke states, with a clear physical interpretation. Our results suggest that
the relation between nonlocality and entanglement of Dicke states is not
monotonous.Comment: 7 pages, 5 figure