825 research outputs found
Classification of Primary Constraints of Quadratic Non-Metricity Theories of Gravity
We perform the ADM decomposition of a five-parameter family of quadratic
non-metricity theories and study their conjugate momenta. After systematically
identifying all possible conditions which can be imposed on the parameters such
that different sets of primary constraints arise, we find that the
five-parametric theory space can be compartmentalized into nine different
sectors, based on the presence or absence of primary constraints. This
classification allows to dismiss certain classes of theories as unphysical and
invites further investigations into the remaining sectors, which may contain
phenomenologically interesting modifications of General Relativity.Comment: 8 pages, 3 tables, 1 figur
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
Hamiltonian Analysis of Gravity and the Failure of the Dirac-Bergmann Algorithm for Teleparallel Theories of Gravity
In recent years, gravity has enjoyed considerable attention in the
literature and important results have been obtained. However, the question of
how many physical degrees of freedom the theory propagates -- and how this
number may depend on the form of the function -- has not been answered
satisfactorily. In this article we show that a Hamiltonian analysis based on
the Dirac-Bergmann algorithm -- one of the standard methods to address this
type of question -- fails. We isolate the source of the failure, show that
other commonly considered teleparallel theories of gravity are affected by the
same problem, and we point out that the number of degrees of freedom obtained
in Phys. Rev. D 106 no. 4, (2022) by K. Hu, T. Katsuragawa, and T. Qui (namely
eight), based on the Dirac-Bergmann algorithm, is wrong. Using a different
approach, we show that the upper bound on the degrees of freedom is seven.
Finally, we propose a more promising strategy for settling this important
question.Comment: 45 pages, 3 figues, Comments are welcom
Device-independent certification of high-dimensional quantum systems
An important problem in quantum information processing is the certification
of the dimension of quantum systems without making assumptions about the
devices used to prepare and measure them, that is, in a device-independent
manner. A crucial question is whether such certification is experimentally
feasible for high-dimensional quantum systems. Here we experimentally witness
in a device-independent manner the generation of six-dimensional quantum
systems encoded in the orbital angular momentum of single photons and show that
the same method can be scaled, at least, up to dimension 13.Comment: REVTeX4, 5 pages, 2 figure
Non-linear extension of non-metricity scalar for MOND
General Relativity enjoys the freedom of different geometrical
interpretations in terms of curvature, torsion or non-metricity. Within this
geometrical trinity, a simpler geometrical formulation of General Relativity
manifests itself in the latter, where gravity is entirely attributed to
non-metricity. In this Letter, we consider non-linear extensions of Coincident
General Relativity for phenomenological applications
on both cosmological as well as galactic scales. The theory not only delivers
dark energy on large scales but also recovers MOND on galactic scales, together
with implications for the early universe cosmology. To the best of our
knowledge, this represents the first relativistic, covariant, and ghost-free
hybrid-formulation of MOND which recovers both, General Relativity and MOND in
the appropriate limits and reconciles expected cosmological behavior. We
further illustrate that previous bimetric formulations of MOND generically
suffer from ghost instabilities and crystalizes as a
unique ghost-free theory.Comment: 6 page
Free-space quantum key distribution by rotation-invariant twisted photons
Twisted photons are photons carrying a well-defined nonzero value of orbital
angular momentum (OAM). The associated optical wave exhibits a helical shape of
the wavefront (hence the name) and an optical vortex at the beam axis. The OAM
of light is attracting a growing interest for its potential in photonic
applications ranging from particle manipulation, microscopy and
nanotechnologies, to fundamental tests of quantum mechanics, classical data
multiplexing and quantum communication. Hitherto, however, all results obtained
with optical OAM were limited to laboratory scale. Here we report the
experimental demonstration of a link for free-space quantum communication with
OAM operating over a distance of 210 meters. Our method exploits OAM in
combination with optical polarization to encode the information in
rotation-invariant photonic states, so as to guarantee full independence of the
communication from the local reference frames of the transmitting and receiving
units. In particular, we implement quantum key distribution (QKD), a protocol
exploiting the features of quantum mechanics to guarantee unconditional
security in cryptographic communication, demonstrating error-rate performances
that are fully compatible with real-world application requirements. Our results
extend previous achievements of OAM-based quantum communication by over two
orders of magnitudes in the link scale, providing an important step forward in
achieving the vision of a worldwide quantum network
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