Transmission of Quantum-Correlated Structured Light in Air-Core Fiber

Distribution of non-classical correlations through optical fibers constitutes a keystone for future quantum networks. Indeed, purely quantum protocols based on entangled systems present communication advantages beyond standard resources. A crucial albeit very 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 of polarization-entangled photons over long distances. Further improvements can be attained by handling and distributing high-dimensional systems, allowing more information content as well as improved security via quantum cryptographic protocols.

Among the different degrees of freedom of light, a promising direction is represented by orbital angular momentum due to its ability 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 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 that 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 team was able to violate tripartite inequalities. Such observed tripartite correlations are generated by both contextual (intra-system) and nonlocal (inter-system) entanglement.

Hybrid entangled state transmission.

The 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 the degrees of freedom. The scalability of this approach widens the range of applicability for the transmission of complex states of light. Look for adoption of high-dimensional entanglement in quantum networks and fiber-based distribution over longer distances of quantum correlated photons.

Read the original research article in the peer-reviewed, open-access journal Advanced Photonics: Cozzolino et al., "Air-core fiber distribution of hybrid vector vortex-polarization entangled states," Adv. Photonics 1(4), 046005 (2019).