6 May 2013 Photonic hybrid multidimensional systems and their application in quantum communication
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Qudits, the d-dimensional extension of qubits, open new perspectives in several fields, from fundamental quantum mechanics to quantum cryptography. Although photon polarization is a privileged choice for qubits encoding, it is not suitable for the physical realization of qudits. However, in order to realize multidimensional quantum systems, other degrees of freedom of single photons such as path or orbital angular momentum are available. When two or more degrees of freedom are exploited simultaneously we refer to "hybrid encoding". It is possible for instance to encode information in a four dimensional (ququart) hybrid space spanned by polarization and a bidimensional orbital angular momentum subspace of a single photon. Here we present how high dimensional hybrid systems can be exploited to overcome a major limitation of quantum communication: the need of a shared reference frame. Indeed the joint action of polarization and orbital angular momentum of hybrid ququarts can be exploited to realize quantum communication without a shared reference frame. We experimentally showed that, by using a proper subspace of hybrid ququart space, it is possible to perform any quantum communication protocol and violate CHSH inequalities without any information about the reference frame orientation of the two parties (except the direction of propagation of the photons). Such feature could find application in satellite based communication schemes.
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V. D'Ambrosio, V. D'Ambrosio, E. Nagali, E. Nagali, L. Marrucci, L. Marrucci, F. Sciarrino, F. Sciarrino, "Photonic hybrid multidimensional systems and their application in quantum communication", Proc. SPIE 8773, Photon Counting Applications IV; and Quantum Optics and Quantum Information Transfer and Processing, 87730P (6 May 2013); doi: 10.1117/12.1518444; https://doi.org/10.1117/12.1518444

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