54 research outputs found
The resource theory of steering
We present an operational framework for Einstein-Podolsky-Rosen steering as a
physical resource. To begin with, we characterize the set of steering
non-increasing operations (SNIOs) --i.e., those that do not create steering--
on arbitrary-dimensional bipartite systems composed of a quantum subsystem and
a black-box device. Next, we introduce the notion of convex steering monotones
as the fundamental axiomatic quantifiers of steering. As a convenient example
thereof, we present the relative entropy of steering. In addition, we prove
that two previously proposed quantifiers, the steerable weight and the
robustness of steering, are also convex steering monotones. To end up with, for
minimal-dimensional systems, we establish, on the one hand, necessary and
sufficient conditions for pure-state steering conversions under stochastic
SNIOs and prove, on the other hand, the non-existence of steering bits, i.e.,
measure-independent maximally steerable states from which all states can be
obtained by means of the free operations. Our findings reveal unexpected
aspects of steering and lay foundations for further resource-theory approaches,
with potential implications in Bell non-locality.Comment: Presentation and structure improve
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
Scalable experimental estimation of multipartite entanglement
We present an efficient experimental estimation of the multipartite
entanglement of mixed quantum states in terms of simple parity measurements.Comment: Three pages, three figure
Nonlocality free wirings and the distinguishability between Bell boxes
Bell nonlocality can be formulated in terms of a resource theory with local-hidden variable models as resourceless objects. Two such theories are known, one built upon local operations assisted by shared randomness (LOSRs) and the other one allowing, in addition, for prior-to-input classical communication. We show that prior communication, although unable to create nonlocality, leads to wirings not only beyond LOSRs but also not contained in a much broader class of (nonlocality-generating) global wirings. Technically, this is shown by proving that it can improve the statistical distinguishability between Bell correlations optimized over all fixed measurement choices. This has implications in nonlocality quantification, and leads us to a natural universal definition of Bell nonlocality measures. To end up with, we also consider the statistical strength of nonlocality proofs. We point out some issues of its standard definition in the resource-theoretic operational framework, and suggest simple fixes for them. Our findings reveal nontrivial features of the geometry of the set of wirings and may have implications in the operational distinguishability of nonlocal behaviors
Noncontextual wirings
Contextuality is a fundamental feature of quantum theory and is necessary for
quantum computation and communication. Serious steps have therefore been taken
towards a formal framework for contextuality as an operational resource.
However, the most important component for a resource theory - a concrete,
explicit form for the free operations of contextuality - was still missing.
Here we provide such a component by introducing noncontextual wirings: a
physically-motivated class of contextuality-free operations with a friendly
parametrization. We characterize them completely for the general case of
black-box measurement devices with arbitrarily many inputs and outputs. As
applications, we show that the relative entropy of contextuality is a
contextuality monotone and that maximally contextual boxes that serve as
contextuality bits exist for a broad class of scenarios. Our results complete a
unified resource-theoretic framework for contextuality and Bell nonlocality
Gapped Two-Body Hamiltonian for continuous-variable quantum computation
We introduce a family of Hamiltonian systems for measurement-based quantum
computation with continuous variables. The Hamiltonians (i) are quadratic, and
therefore two body, (ii) are of short range, (iii) are frustration-free, and
(iv) possess a constant energy gap proportional to the squared inverse of the
squeezing. Their ground states are the celebrated Gaussian graph states, which
are universal resources for quantum computation in the limit of infinite
squeezing. These Hamiltonians constitute the basic ingredient for the adiabatic
preparation of graph states and thus open new venues for the physical
realization of continuous-variable quantum computing beyond the standard
optical approaches. We characterize the correlations in these systems at
thermal equilibrium. In particular, we prove that the correlations across any
multipartition are contained exactly in its boundary, automatically yielding a
correlation area law.Comment: 4 pages, one figure. New version: typos corrected, one reference
added. To appear in PR
Robust-fidelity atom-photon entangling gates in the weak-coupling regime
We describe a simple entangling principle based on the scattering of photons
off single emitters in one-dimensional waveguides (or extremely-lossy
cavities). The scheme can be applied to photonic qubits encoded in polarization
or time-bin, and features a filtering mechanism that works effectively as a
built-in error-correction directive. This automatically maps imperfections from
weak couplings, atomic decay into undesired modes, frequency mismatches, or
finite bandwidths of the incident photonic pulses, into heralded losses instead
of infidelities. The scheme is thus adequate for high-fidelity maximally
entangling gates even in the weak-coupling regime. These, in turn, can be
directly applied to store and retrieve photonic-qubit states, thereby
completing an atom-photon interface toolbox, or to sequential measurement-based
quantum computations with atomic memories.Comment: 5 pages, 2 figure
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