118 research outputs found
Looking for symmetric Bell inequalities
Finding all Bell inequalities for a given number of parties, measurement
settings, and measurement outcomes is in general a computationally hard task.
We show that all Bell inequalities which are symmetric under the exchange of
parties can be found by examining a symmetrized polytope which is simpler than
the full Bell polytope. As an illustration of our method, we generate 238885
new Bell inequalities and 1085 new Svetlichny inequalities. We find, in
particular, facet inequalities for Bell experiments involving two parties and
two measurement settings that are not of the
Collins-Gisin-Linden-Massar-Popescu type.Comment: Joined the associated website as an ancillary file, 17 pages, 1
figure, 1 tabl
Proposal for witnessing non-classical light with the human eye
We give a complete proposal showing how to detect the non-classical nature of
photonic states with naked eyes as detectors. The enabling technology is a
sub-Poissonian photonic state that is obtained from single photons,
displacement operations in phase space and basic non-photon-number-resolving
detectors. We present a detailed statistical analysis of our proposal including
imperfect photon creation and detection and a realistic model of the human eye.
We conclude that a few tens of hours are sufficient to certify non-classical
light with the human eye with a p-value of 10%.Comment: 9 pages, 5 figures, accepted versio
Noisy pre-processing facilitating a photonic realisation of device-independent quantum key distribution
Device-independent quantum key distribution provides security even when the
equipment used to communicate over the quantum channel is largely
uncharacterized. An experimental demonstration of device-independent quantum
key distribution is however challenging. A central obstacle in photonic
implementations is that the global detection efficiency, i.e., the probability
that the signals sent over the quantum channel are successfully received, must
be above a certain threshold. We here propose a method to significantly relax
this threshold, while maintaining provable device-independent security. This is
achieved with a protocol that adds artificial noise, which cannot be known or
controlled by an adversary, to the initial measurement data (the raw key).
Focusing on a realistic photonic setup using a source based on spontaneous
parametric down conversion, we give explicit bounds on the minimal required
global detection efficiency.Comment: 5+16 pages, 4 figure
Bell inequalities for three systems and arbitrarily many measurement outcomes
We present a family of Bell inequalities for three parties and arbitrarily
many outcomes, which can be seen as a natural generalization of the Mermin Bell
inequality. For a small number of outcomes, we verify that our inequalities
define facets of the polytope of local correlations. We investigate the quantum
violations of these inequalities, in particular with respect to the Hilbert
space dimension. We provide strong evidence that the maximal quantum violation
can only be reached using systems with local Hilbert space dimension exceeding
the number of measurement outcomes. This suggests that our inequalities can be
used as multipartite dimension witnesses.Comment: v1 6 pages, 4 tables; v2 Published version with minor typos correcte
Comparing different approaches for generating random numbers device-independently using a photon pair source
What is the most efficient way to generate random numbers
device-independently using a photon pair source based on spontaneous parametric
down conversion (SPDC)? We consider this question by comparing two
implementations of a detection-loophole-free Bell test. In particular, we study
in detail a scenario where a heralded single photon source (HSPS) is used to
herald path-entangled states, i.e. entanglement between two spatial modes
sharing a single photon and where non-locality is revealed using photon
counting preceded by small displacement operations. We start by giving a
theoretical description of such a measurement. We then show how to optimize the
Bell-CHSH violation through a non-perturbative calculation, taking the main
experimental imperfections into account. We finally bound the amount of
randomness that can be extracted and compare it to the one obtained with the
conventional scenario using photon pairs entangled e.g. in polarization and
analyzed through photon counting. While the former requires higher overall
detection efficiencies, it is far more efficient in terms of both the entropy
per experimental run and the rate of random bit generation.Comment: 12 pages, 5 figure
New Examples of Kochen-Specker Type Configurations on Three Qubits
A new example of a saturated Kochen-Specker (KS) type configuration of 64
rays in 8-dimensional space (the Hilbert space of a triple of qubits) is
constructed. It is proven that this configuration has a tropical dimension 6
and that it contains a critical subconfiguration of 36 rays. A natural
multicolored generalisation of the Kochen-Specker theory is given based on a
concept of an entropy of a saturated configuration of rays.Comment: 24 page
Physical characterization of quantum devices from nonlocal correlations
In the device-independent approach to quantum information theory, quantum systems are re- garded as black boxes which, given an input (the measurement setting), return an output (the measurement result). These boxes are then treated regardless of their actual internal working. In this paper, we develop SWAP, a theoretical concept which, in combination with the tool of semi- definite methods for the characterization of quantum correlations, allows us to estimate physical properties of the black boxes from the observed measurement statistics. We find that the SWAP tool provides bounds orders of magnitude better than previously-known techniques (e.g.: for a CHSH violation larger than 2.57, SWAP predicts a singlet fidelity greater than 70%). This method also allows us to deal with hitherto intractable cases such as robust device-independent self-testing of non-maximally entangled two-qutrit states in the CGLMP scenario (for which Jordan’s Lemma does not apply) and the device-independent certification of entangled measurements. We further apply the SWAP method to relate nonlocal correlations to work extraction and quantum dimensionality, hence demonstrating that this tool can be used to study a wide variety of properties relying on the sole knowledge of accessible statistics
Practical private database queries based on a quantum key distribution protocol
Private queries allow a user Alice to learn an element of a database held by
a provider Bob without revealing which element she was interested in, while
limiting her information about the other elements. We propose to implement
private queries based on a quantum key distribution protocol, with changes only
in the classical post-processing of the key. This approach makes our scheme
both easy to implement and loss-tolerant. While unconditionally secure private
queries are known to be impossible, we argue that an interesting degree of
security can be achieved, relying on fundamental physical principles instead of
unverifiable security assumptions in order to protect both user and database.
We think that there is scope for such practical private queries to become
another remarkable application of quantum information in the footsteps of
quantum key distribution.Comment: 7 pages, 2 figures, new and improved version, clarified claims,
expanded security discussio
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