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Quantum key distribution (QKD) has long been a promising area for applicationof quantum effects toward solving real-world problems. But two major obstacleshave stood in the way of widespread applications: low secure key generationrates and short achievable operating distances. In this paper, a new physicalmechanism for dealing with the first of these problems is proposed: interplaybetween different degrees of freedom in a hyperentangled system (parametricdown conversion) is used to increase the Hilbert space dimension available forkey generation while maintaining security. Polarization-based Bell testsprovide security checking, while orbital angular momentum (OAM) and totalangular momentum (TAM) provide higher key generation rate. Whether to measureTAM or OAM is decided randomly on each trial. The concurrent non-commutativityof TAM with OAM and polarization provides the physical basis for quantumsecurity. TAM measurements link polarization to OAM, so that if the legitimateparticipants measure OAM while the eavesdropper measures TAM (or vice-versa),then polarization entanglement is lost, revealing the eavesdropper. In contrastto other OAM-based QKD methods, complex active switching between OAM bases isnot required; instead, passive switching by beam splitters combined with muchsimpler active switching between polarization bases makes implementation athigh OAM more practical.

High Capacity Quantum Key Distribution via Hyper-Entangled Degrees of Freedom