Multiplexing is a strategy to augment the transmission capacity of a communication system. It consists of combining multiple signals over the same data channel and it has been very successful in classical communications. However, the use of enhanced channels has only reached limited practicality in quantum communications (QC) as it requires the complex manipulation of quantum systems of higher dimensions. Considerable effort is being made towards QC using high-dimensional quantum systems encoded into the transverse momentum of single photons but, so far, no approach has been proven to be fully compatible with the existing telecommunication infrastructure. Here, we overcome such a technological challenge and demonstrate a stable and secure high-dimensional decoy-state quantum key distribution session over a 0.3 km long multicore optical fiber. The high-dimensional quantum states are defined in terms of the multiple core modes available for the photon transmission over the fiber, and the decoy-state analysis demonstrates that our technique enables a positive secret key generation rate up to 25 km of fiber propagation. Finally, we show how our results build up towards a high-dimensional quantum network composed of free-space and fiber based links.
The results of space-like separated measurements are independent of distant measurement settings, a property one might call two-way no-signalling. In contrast, time-like separated measurements are only one-way no-signalling since the past is independent of the future but not vice-versa. For this reason some temporal correlations that are formally identical to non-classical spatial correlations can still be modelled classically. We propose a new formulation of Bell's theorem for temporal correlations, namely we define non-classical temporal correlations as the ones which cannot be simulated by propagating in time the classical information content of a quantum system given by the Holevo bound. We first show that temporal correlations between results of any projective quantum measurements on a qubit can be simulated classically. Then we present a sequence of POVM measurements on a single m-level quantum system that cannot be explained by propagating in time an m-level classical system and using classical computers with unlimited memory.Introduction. The violation of a Bell inequality [1][2][3] demonstrates that the outcomes of an experiment have contradicted a set of well defined classical intuitions. Quantum mechanics allows correlations between spacelike separated parties that have no explanation in terms of a hidden variable model, i.e. they cannot be reproduced with the help of classical computers running preagreed algorithms. However, when correlations are generated in a temporal scenario, by a sequence of time-like separated measurements, it is more difficult to demonstrate their non-classical nature. The causal structure of physics implies only one-way no-signalling, namely the impossibility of sending communication backwards in time. The only bound on forward signalling is the information capacity of the physical system.Here we analyse a single quantum system measured at n points in time and consider to what extent one can prove that the temporal correlations between these measurement outcomes could not be generated by a classical system. We assume an idealization, in which the m-level physical system carries no hidden degrees of freedom. In this case the classical information capacity of the system is log 2 m; known as the Holevo bound [4].The previous approach to demonstrate non-classicality of temporal quantum correlations are so called "temporal Bell inequalities" [5][6][7][8][9][10][11][12][13][14]. One of the problems with this approach, stated in [13], is that the classical assumptions behind the temporal inequalities, which are realism and non-invasiveness, were originally chosen to test the quantumness of a temporal evolution of macroscopic quantum systems [5]. As such, they do not provide a convincing test of quantumness in the case of a single evolving sys-
Attractive ultracold fermions trapped in a one-dimensional periodically shaken optical lattice are considered. For an appropriate resonant shaking, a dimerized structure emerges for which the system realizes paradigmatic physics described by the Rice-Mele model. The emergent nature of the system together with density fluctuations or controlled modifications of lattice filling allow for the creation of defects. Those defects lead to topologically protected localized modes carrying the fractional particle number. Their possible experimental signatures are discussed.
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