710 research outputs found
Maximal randomness expansion from steering inequality violations using qudits
We consider the generation of randomness based upon the observed violation of
an Einstein-Podolsky-Rosen (EPR) steering inequality, known as one-sided
device-independent randomness expansion. We show that in the simplest scenario
-- involving only two parties applying two measurements with outcomes each
-- that there exist EPR steering inequalities whose maximal violation certifies
the maximal amount of randomness, equal to log(d) bits. We further show that
all pure partially entangled full-Schmidt-rank states in all dimensions can
achieve maximal violation of these inequalities, and thus lead to maximal
randomness expansion in the one-sided device-independent setting. More
generally, the amount of randomness that can be certified is given by a
semidefinite program, which we use to study the behaviour for non-maximal
violations of the inequalities.Comment: 6 pages, 1 figur
Loss-tolerant EPR steering for arbitrary dimensional states: joint measurability and unbounded violations under losses
We show how to construct loss-tolerant linear steering inequalities using a
generic set of von Neumann measurements that are violated by -dimensional
states, and that rely only upon a simple property of the set of measurements
used (the maximal overlap between measurement directions). Using these
inequalities we show that the critical detection efficiency above which von
Neumann measurements can demonstrate steering is . We show furthermore
that using our construction and high dimensional states allows for steering
demonstrations which are also highly robust to depolarising noise and produce
unbounded violations in the presence of loss. Finally, our results provide an
explicit means to certify the non-joint measurability of any set of inefficient
von Neuman measurements.Comment: 4+3 pages. v2: title changed. Results on unbounded violation of
steering inequalities added. Accepted by PR
Measurement-device-independent entanglement and randomness estimation in quantum networks
Detection of entanglement in quantum networks consisting of many parties is
one of the important steps towards building quantum communication and
computation networks. We consider a scenario where the measurement devices used
for this certification are uncharacterised. In this case, it is well known that
by using quantum states as inputs for the measurement devices it is possible to
detect any entangled state (a situation known as measurement device-independent
entanglement witnessing). Here we go beyond entanglement detection and provide
methods to estimate the amount of entanglement in a quantum network. We also
consider the task of randomness certification and show that randomness can be
certified in a variety of cases, including single-partite experiments or setups
using only separable states.Comment: 10 pages, 1 figure, close to published versio
Causal hierarchy of multipartite Bell nonlocality
As with entanglement, different forms of Bell nonlocality arise in the
multipartite scenario. These can be defined in terms of relaxations of the
causal assumptions in local hidden-variable theories. However, a
characterisation of all the forms of multipartite nonlocality has until now
been out of reach, mainly due to the complexity of generic multipartite causal
models. Here, we employ the formalism of Bayesian networks to reveal
connections among different causal structures that make a both practical and
physically meaningful classification possible. Our framework holds for
arbitrarily many parties. We apply it to study the tripartite scenario in
detail, where we fully characterize all the nonlocality classes. Remarkably, we
identify new highly nonlocal causal structures that cannot reproduce all
quantum correlations. This shows, to our knowledge, the strongest form of
quantum multipartite nonlocality known to date. Finally, as a by-product
result, we derive a non-trivial Bell-type inequality with no quantum violation.
Our findings constitute a significant step forward in the understanding of
multipartite Bell nonlocality and open several venues for future research.Comment: 6 pages + appendix, 3 figures, 3 tables. Minor errors corrected,
discovery of strongest form of quantum multipartite non-locality known so far
added. v3: text improved. v4: Accepted by Quantu
Quantifying Einstein-Podolsky-Rosen steering
Einstein-Podolsky-Rosen (EPR) steering is a form of bipartite quantum
correlation that is intermediate between entanglement and Bell nonlocality. It
allows for entanglement certification when the measurements performed by one of
the parties are not characterised (or are untrusted) and has applications in
quantum key distribution. Despite its foundational and applied importance, EPR
steering lacks a quantitative assessment. Here we propose a way of quantifying
this phenomenon and use it to study the steerability of several quantum states.
In particular we show that every pure entangled state is maximally steerable,
the projector onto the anti-symmetric subspace is maximally steerable for all
dimensions, we provide a new example of one-way steering, and give strong
support that states with positive-partial-transposition are not steerable.Comment: 9 pages, 1 figure. v2: One example (ex. (iv)) removed. One appendix
(E) and one reference ([28]) added. V3: new example of one-way steering
included, typos corrected, new reference
All sets of incompatible measurements give an advantage in quantum state discrimination
Some quantum measurements can not be performed simultaneously, i.e. they are
incompatible. Here we show that every set of incompatible measurements provides
an advantage over compatible ones in a suitably chosen quantum state
discrimination task. This is proven by showing that the Robustness of
Incompatibility, a quantifier of how much noise a set of measurements tolerates
before becoming compatible, has an operational interpretation as the advantage
in an optimally chosen discrimination task. We also show that if we take a
resource-theory perspective of measurement incompatibility, then the guessing
probability in discrimination tasks of this type forms a complete set of
monotones that completely characterize the partial order in the resource
theory. Finally, we make use of previously known relations between measurement
incompatibility and Einstein-Podolsky-Rosen steering to also relate the later
with quantum state discrimination.Comment: 10 pages, no figure
Estimating entanglement in teleportation experiments
Quantum state teleportation is a protocol where a shared entangled state is
used as a quantum channel to transmit quantum information between distinct
locations. Here we consider the task of estimating entanglement in
teleportation experiments. We show that the data accessible in a teleportation
experiment allows to put a lower bound on some entanglement measures, such as
entanglement negativity and robustness. Furthermore, we show cases in which the
lower bounds are tight. The introduced lower bounds can also be interpreted as
quantifiers of the nonclassicality of a teleportation experiment. Thus, our
findings provide a quantitative relation between teleportation and
entanglement.Comment: The title is changed and the manuscript is significantly
restructured. Codes available at
https://github.com/paulskrzypczyk/nonclassicalteleportation/blob/master/Quantifying%20teleportation.ipyn
All entangled states can demonstrate non-classical teleportation
Quantum teleportation, the process by which Alice can transfer an unknown
quantum state to Bob by using pre-shared entanglement and classical
communication, is one of the cornerstones of quantum information. The standard
benchmark for certifying quantum teleportation consists in surpassing the
maximum average fidelity between the teleported and the target states that can
be achieved classically. According to this figure of merit, not all entangled
states are useful for teleportation. Here we propose a new benchmark that uses
the full information available in a teleportation experiment and prove that all
entangled states can implement a quantum channel which can not be reproduced
classically. We introduce the idea of non-classical teleportation witness to
certify if a teleportation experiment is genuinely quantum and discuss how to
quantify this phenomenon. Our work provides new techniques for studying
teleportation that can be immediately applied to certify the quality of quantum
technologies.Comment: v5: correction made (Tau_R is proportional to E_R in the case of a
partial Bell state measurement). Main results untouche
Analysis of a proposal for a realistic loophole-free Bell test with atom-light entanglement
The violation of Bell inequalities where both detection and locality
loopholes are closed is crucial for device independent assessments of quantum
information. While of technological nature, the simultaneous closing of both
loopholes still remains a challenge. In Nat. Commun. 4:2104(2013), a realistic
setup to produce an atom-photon entangled state that could reach a loophole
free Bell inequality violation within current experimental technology was
proposed. Here we improve the analysis of this proposal by giving an analytical
treatment that shows that the state proposed in Nat. Commun. 4:2104(2013) could
in principle violate a Bell inequality for arbitrarily low photodetection
efficiency. Moreover, it is also able to violate a Bell inequality considering
only atomic and homodyne measurements eliminating the need to consider
inefficient photocounting measurements. In this case, the maximum
Clauser-Horne-Shimony-Holt (CHSH) inequality violation achievable is 2.29, and
the minimum transmission required for violation is about 68%. Finally, we show
that by postselecting on an atomic measurement, one can engineer superpositions
of coherent states for various coherent state amplitudes.Comment: 7 pages, 6 figures, to appear in Phys. Rev.
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