109 research outputs found
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
Robustness of Measurement, discrimination games and accessible information
We introduce a way of quantifying how informative a quantum measurement is,
starting from a resource-theoretic perspective. This quantifier, which we call
the robustness of measurement, describes how much `noise' must be added to a
measurement before it becomes completely uninformative. We show that this
geometric quantifier has operational significance in terms of the advantage the
measurement provides over guessing at random in an suitably chosen state
discrimination game. We further show that it is the single-shot generalisation
of the accessible information of a certain quantum-to-classical channel. Using
this insight, we also show that the recently-introduced robustness of coherence
is the single-shot generalisation of the accessible information of an ensemble.
Finally we discuss more generally the connection between robustness-based
measures, discrimination problems and single-shot information theory.Comment: 10 pages, 1 figur
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
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
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
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
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
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
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
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