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.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information
field. In this paper we report some recent results regarding the adoption of higher dimensional quantum
states encoded in the polarization and orbital angular momentum for quantum information and cryptographic
processing.
The orbital angular momentum carried by single photons represents a promising resource in the quantum information
field. In this paper we report the characterization in the quantum regime of a recently introduced
optical device, known as q-plate. Exploiting the spin-orbit coupling that takes place in the q-plate, it is possible
to transfer coherently the information from the polarization to the orbital angular momentum degree of freedom,
and viceversa. Hence the q-plate provides a reliable bi-directional interface between polarization and orbital
angular momentum. As a first paradigmatic demonstration of the q-plate properties, we have carried out the
first experimental Hong-Ou-mandel effect purely observed in the orbital angular momentum degree of freedom.
We investigate the multiphoton states generated by high-gain optical parametric amplification of a single injected
photon, polarization encoded as a "qubit". The experiment configuration exploits the optimal phase-covariant
cloning in the high gain regime. The interference fringe pattern showing the non local transfer of coherence
between the injected qubit and the mesoscopic amplified output field involving up to 4000 photons has been
investigated. A probabilistic new method to extract full information about the multiparticle output wavefunction
has been implemented. This technique can be adopted to test the entanglement between a microscopic system
and a macro one.
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