1,279 research outputs found
Bell-type inequalities for non-local resources
We present bipartite Bell-type inequalities which allow the two partners to
use some non-local resource. Such inequality can only be violated if the
parties use a resource which is more non-local than the one permitted by the
inequality. We introduce a family of N-inputs non-local machines, which are
generalizations of the well-known PR-box. Then we construct Bell-type
inequalities that cannot be violated by strategies that use one these new
machines. Finally we discuss implications for the simulation of quantum states.Comment: 8 pages, 4 figure
Bell nonlocality and Bayesian game theory
We discuss a connection between Bell nonlocality and Bayesian games. This
link offers interesting perspectives for Bayesian games, namely to allow the
players to receive advice in the form of nonlocal correlations, for instance
using entangled quantum particles or more general no-signaling boxes. The
possibility of having such 'nonlocal advice' will lead to novel joint
strategies, impossible to achieve in the classical setting. This implies that
quantum resources, or more general no-signaling resources, offer a genuine
advantage over classical ones. Moreover, some of these strategies can represent
equilibrium points, leading to the notion of quantum/no-signaling Nash
equilibrium. Finally we describe new types of question in the study of
nonlocality, namely the consideration of non-local advantage when there is a
set of Bell expressions.Comment: 7 pages, 3 figure
Semi-device-independent security of one-way quantum key distribution
By testing nonlocality, the security of entanglement-based quantum key
distribution (QKD) can be enhanced to being 'device-independent'. Here we ask
whether such a strong form of security could also be established for one-way
(prepare and measure) QKD. While fully device-independent security is
impossible, we show that security can be guaranteed against individual attacks
in a semi-device-independent scenario. In the latter, the devices used by the
trusted parties are non-characterized, but the dimensionality of the quantum
systems used in the protocol is assumed to be bounded. Our security proof
relies on the analogies between one-way QKD, dimension witnesses and
random-access codes.Comment: 5 pages, 1 figur
Couplers for Non-Locality Swapping
Studying generalized non-local theories brings insight to the foundations of
quantum mechanics. Here we focus on non-locality swapping, the analogue of
quantum entanglement swapping. In order to implement such a protocol, one needs
a coupler that performs the equivalent of quantum joint measurements on
generalized `box-like' states. Establishing a connection to Bell inequalities,
we define consistent couplers for theories containing an arbitrary amount of
non-locality, which leads us to introduce the concepts of perfect and minimal
couplers. Remarkably, Tsirelson's bound for quantum non-locality naturally
appears in our study.Comment: 16 pages, 3 figure
Small quantum absorption refrigerator in the transient regime: time scales, enhanced cooling and entanglement
A small quantum absorption refrigerator, consisting of three qubits, is
discussed in the transient regime. We discuss time scales for coherent
dynamics, damping, and approach to the steady state, and we study cooling and
entanglement. We observe that cooling can be enhanced in the transient regime,
in the sense that lower temperatures can be achieved compared to the
steady-state regime. This is a consequence of coherent dynamics, but can occur
even when this dynamics is strongly damped by the dissipative thermal
environment, and we note that precise control over couplings or timing is not
needed to achieve enhanced cooling. We also show that the amount of
entanglement present in the refrigerator can be much larger in the transient
regime compared to the steady-state. These results are of relevance to future
implementations of quantum thermal machines.Comment: 8 pages, 5 figure
Dimension witnesses and quantum state discrimination
Dimension witnesses allow one to test the dimension of an unknown physical
system in a device-independent manner, that is, without placing assumptions
about the functioning of the devices used in the experiment. Here we present
simple and general dimension witnesses for quantum systems of arbitrary Hilbert
space dimension. Our approach is deeply connected to the problem of quantum
state discrimination, hence establishing a strong link between these two
research topics. Finally, our dimension witnesses can distinguish between
classical and quantum systems of the same dimension, making them potentially
useful for quantum information processing.Comment: 5 page
A short note on passivity, complete passivity and virtual temperatures
We give a simple and intuitive proof that the only states which are
completely passive, i.e. those states from which work cannot be extracted even
with infinitely many copies, are Gibbs states at positive temperatures. The
proof makes use of the idea of virtual temperatures, i.e. the association of
temperatures to pairs of energy levels (transitions). We show that (i) passive
states are those where every transition is at a positive temperature, and (ii)
completely passive states are those where every transition is at the same
positive temperature.Comment: 3 pages, no figures. v2: Published versio
Sequential random access codes and self-testing of quantum measurement instruments
Quantum Random Access Codes (QRACs) are key tools for a variety of protocols
in quantum information theory. These are commonly studied in
prepare-and-measure scenarios in which a sender prepares states and a receiver
measures them. Here, we consider a three-party prepare-transform-measure
scenario in which the simplest QRAC is implemented twice in sequence based on
the same physical system. We derive optimal trade-off relations between the two
QRACs. We apply our results to construct semi-device independent self-tests of
quantum instruments, i.e. measurement channels with both a classical and
quantum output. Finally, we show how sequential QRACs enable inference of upper
and lower bounds on the sharpness parameter of a quantum instrument
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