High-dimensional quantum key distribution by coherence modulation and orbital angular momentum at the photon-counting level

Abstract

Quantum Key Distribution (QKD) systems usually exploit the polarization of light to encode bit values, thus limiting to 1 bit the amount of information carried by each photon and placing serious limits on the error rates tolerated. Here we propose the use of two mutually unbiased bases for high-dimensional QKD that exploit the transverse spatial structure and coherence properties of the light field, allowing for the transfer of more than 1 bit per photon. Our proposed method employs coherence modulation with an orthonormal basis of time delays (TD) and the corresponding mutually unbiased basis of wave trains (WT). We construct the mutually unbiased basis set WT using a linear combination of orbital angular momentum OAM modes. Through the use of a high-dimensional alphabet encoded in the TD and WT bases, we achieve a high channel capacity of bits per inspected photon. In addition to exhibiting increased channel capacity, multidimensional QKD systems based on spatiotemporal encoding may be more resilient against intercept-resend eavesdropping attacks. Numerical simulations are presented as tests of the proposed QKD system. Experiments remain to be conducted to verify the concept.