Quantum cryptography with structured light

Abstract

Contemporary communication techniques generally involve encrypting sensitive data by means of mathematically complex algorithms and then distributing it and the accompanying digital decipher keys along classical channels. However, advances in current computing power pose a threat to the security of these methods, leaving them susceptible to being compromised without being noticed. This has led to an unprecedented surge in the exploration of quantum protocols, where quantum mechanical principles, such as the no-cloning theorem and non-locality, offer unconditional security against an eavesdropper. A current limitation of these protocols is the slower communication rates compared to popular classical implementations. Much focus has been on systems where information is encoded in a single degree of freedom (e.g. polarisation), however we demonstrate that high-dimensional protocols have the potential to overcome the slow rate limitation, offering an increased encoding capacity and a more robust solution. Spatially structured modes of light carrying orbital angular momentum have become the forerunners for realising high-dimensional quantum protocols. Here, we investigate the high-dimensional generation, manipulation and detection of structured light and in doing so we realise two important applications- the generation and transmission of encryption keys - the key steps of a quantum cryptographic system. The first application is a quantum random number generator certified by the random outcomes of projective measurements on path superpositions that we engineer using digital holography. We extend our approach to realise a novel random number generator based on the orbital angular momentum entanglement of spontaneous parametric down-converted states. We show that the digital control offered by our setup can be used to compensate for any bias present within system or to generate random numbers with a desired probability distributions. For the second application, we demonstrate the potential of using the high-dimensional encoding space offered by spin-orbit coupled states for quantum secret sharing. We realise the highest number of participants and dimensions thus far and in the process surpass the 1 bit per photon limit of two-dimensional protocols