57 research outputs found
Experimental bilocality violation without shared reference frames
Non-classical correlations arising in complex quantum networks are attracting
growing interest, both from a fundamental perspective and for potential
applications in information processing. In particular, in an entanglement
swapping scenario a new kind of correlations arise, the so-called nonbilocal
correlations that are incompatible with local realism augmented with the
assumption that the sources of states used in the experiment are independent.
In practice, however, bilocality tests impose strict constraints on the
experimental setup and in particular to presence of shared reference frames
between the parties. Here, we experimentally address this point showing that
false positive nonbilocal quantum correlations can be observed even though the
sources of states are independent. To overcome this problem, we propose and
demonstrate a new scheme for the violation of bilocality that does not require
shared reference frames and thus constitute an important building block for
future investigations of quantum correlations in complex networks.Comment: 10 page
Experimental investigation on the geometry of GHz states
Nonclassical correlations arising in complex quantum networks are attracting growing interest, both from afundamental perspective and for potential applications in information processing. In particular, in an entanglementswapping scenario a new kind of correlations arise, the so-called nonbilocal correlations that are incompatible withlocal realism augmented with the assumption that the sources of states used in the experiment are independent.In practice, however, bilocality tests impose strict constraints on the experimental setup and in particular to thepresence of shared reference frames between the parties. Here, we experimentally address this point showing thatfalse positive nonbilocal quantum correlations can be observed even though the sources of states are independent.To overcome this problem, we propose and demonstrate a scheme for the violation of bilocality that does notrequire shared reference frames and thus constitutes an important building block for future investigations ofquantum correlations in complex network
Maximal qubit violation of n-locality inequalities in a star-shaped quantum network
Bellʼs theorem was a cornerstone for our understanding of quantum theory and the establishment ofBell non-locality played a crucial role in the development of quantum information. Recently, itsextension to complex networks has been attracting growing attention, but a deep characterization ofquantum behavior is still missing for this novel context. In this work we analyze quantum correlationsarising in the bilocality scenario, that is a tripartite quantum network where the correlations betweenthe parties are mediated by two independent sources of states. First, we prove that non-bilocalcorrelations witnessed through a Bell-state measurement in the central node of the network form asubset of those obtainable by means of a local projective measurement. This leads us to derive themaximal violation of the bilocality inequality that can be achieved by arbitrary two-qubit quantumstates and arbitrary local projective measurements. We then analyze in details the relation between theviolation of the bilocality inequality and the CHSH inequality. Finally, we show how our method canbe extended to then-locality scenario consisting ofntwo-qubit quantum states distributed amongn1+nodes of a star-shaped networ
Experimental device-independent certified randomness generation with an instrumental causal structure
The intrinsic random nature of quantum physics offers novel tools for the
generation of random numbers, a central challenge for a plethora of fields.
Bell non-local correlations obtained by measurements on entangled states allow
for the generation of bit strings whose randomness is guaranteed in a
device-independent manner, i.e. without assumptions on the measurement and
state-generation devices. Here, we generate this strong form of certified
randomness on a new platform: the so-called instrumental scenario, which is
central to the field of causal inference. First, we theoretically show that
certified random bits, private against general quantum adversaries, can be
extracted exploiting device-independent quantum instrumental-inequality
violations. To that end, we adapt techniques previously developed for the Bell
scenario. Then, we experimentally implement the corresponding
randomness-generation protocol using entangled photons and active feed-forward
of information. Moreover, we show that, for low levels of noise, our protocol
offers an advantage over the simplest Bell-nonlocality protocol based on the
Clauser-Horn-Shimony-Holt inequality.Comment: Modified Supplementary Information: removed description of extractor
algorithm introduced by arXiv:1212.0520. Implemented security of the protocol
against general adversarial attack
Experimental device-independent tests of quantum channels
Quantum tomography is currently the mainly employed method to assess the
information of a system and therefore plays a fundamental role when trying to
characterize the action of a particular channel. Nonetheless, quantum
tomography requires the trust that the devices used in the laboratory perform
state generation and measurements correctly. This work is based on the
theoretical framework for the device-independent inference of quantum channels
that was recently developed and experimentally implemented with superconducting
qubits in [Dall'Arno, Buscemi, Vedral, arXiv:1805.01159] and [Dall'Arno,
Brandsen, Buscemi, PRSA 473, 20160721 (2017)]. Here, we present a complete
experimental test on a photonic setup of two device-independent quantum
channels falsification and characterization protocols to analyze, validate, and
enhance the results obtained by conventional quantum process tomography. This
framework has fundamental implications in quantum information processing and
may also lead to the development of new methods removing the assumptions
typically taken for granted in all the previous protocols
Postselection-Loophole-Free Bell Test Over an Installed Optical Fiber Network
Device-independent (DI) quantum communication will require a loophole-free
violation of Bell inequalities. In typical scenarios where line-of-sight
between the communicating parties is not available, it is convenient to use
energy-time entangled photons due to intrinsic robustness while propagating
over optical fibers. Here we show an energy-time Clauser-Horne-Shimony-Holt
Bell inequality violation with two parties separated by 3.7 km over the
deployed optical fiber network belonging to the University of Concepci\'on in
Chile. Remarkably, this is the first Bell violation with spatially separated
parties that is free of the post-selection loophole, which affected all
previous in-field long-distance energy-time experiments. Our work takes a
further step towards a fiber-based loophole-free Bell test, which is highly
desired for secure quantum communication due to the widespread existing
telecommunication infrastructure.Comment: 5 pages, 3 figures. Matches published versio
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