Distribution of non-classical correlations through optical fibers constitutes a keystone for future quantum networks. Indeed, purely quantum protocols based on entangled systems can disclose communication advantages with respect to what can be achieved with classical resources. A crucial challenging task in this context is then the capability to perform long distance transmission of photon states by preserving their quantum correlations. In the last few years, several demonstrations have been performed showing the transmission over long distances of polarization-entangled photons. Further improvements can be disclosed by handling and distributing high-dimensional systems, allowing to enlarge the information content as well as to improve the security in quantum cryptographic protocols.
Among the different degrees of freedom of the light, a promising direction is represented by orbital angular momentum due to its capability to encode quantum states in higher dimensions and to be employed for generation of hybrid entangled systems (i.e. entanglement between different degrees of freedom). A relevant example of such hybrid states is provided by vector vortex beams, whose peculiar property resides in the quantum correlations between the polarization and the spatial profile of the same photon. However, application of such states in long distance quantum communication protocols is still limited. Indeed, while fiber-based transmission of polarization encoded states is an established task, transmission of light with complex spatial profiles requires the development of special techniques and fibers to be accomplished. Fiber transmission of quantum states carrying orbital angular momentum is a fairly new research field, and experimental demonstrations of the capability to preserve hybrid entanglement after fiber propagation are still lacking.
An international research team from Sapienza University of Rome and Technical University of Denmark has recently provided a step forward in this direction by demonstrating the capability to distribute hybrid vector vortex-polarization entangled photon pairs at telecom wavelength through a specially designed air-core fiber, which allows the propagation of orbital angular momentum. In this case the polarization of a photon is entangled with the vector-vortex state of a second photon, that is sent and transmitted through an air-core fiber. The high fidelities of the distributed entangled states are certified by quantum state tomography in the polarization-orbital angular momentum space. Violation of Bell’s inequalities demonstrates the conservation of quantum correlations after fiber propagation. Furthermore, since the state of two photons involves three different qubits encoded in multiple degrees of freedom, the authors were able to violate tripartite inequalities. Such observed tripartite correlations are generated by both contextual (intra-system) and nonlocal (inter-system) entanglement. The research results are published in Advanced Photonics, Volume 1, No. 4, 2019. (Daniele Cozzolino, Emanuele Polino, Mauro Valeri, Gonzalo Carvacho, Davide Bacco, Nicolò Spagnolo, Leif K. Oxenløwe, Fabio Sciarrino. Air-core fiber distribution of hybrid vector vortex-polarization entangled states.)
The obtained results constitute a building block for future distribution of hybrid entanglement involving polarization and orbital angular momentum and, in general, of high-dimensional quantum states through suitably designed fibers able to preserve correlations in these degrees of freedom. The scalability of this approach widens the range of applicability for the transmission of complex states of light. These include adoption of high-dimensional entanglement in quantum networks, fiber-based distribution over longer distances of quantum correlated photons and interfacing integrated circuits with fibers supporting propagation of orbital angular momentum.
Schematic diagram of polarization-orbital angular momentum hybrid entangled fiber transmission
