Analogy of free-space quantum key distribution using spatial modes of light: scaling up the distance and the dimensionality

While quantum key distribution (QKD) shows much potential as a solution to the looming threat that quantum computers place on today’s encryption protocols, efficient and robust systems that operate in free space are still limited. In this paper, we present a classical light analogy of QKD using spatial modes of light, which can provide a higher bit per photon rate than the more commonly used polarization state encoding.We start by demonstrating the BB84 protocol indoors, where we achieve a secure key rate of 1.55 bits per sifted photon. Then, using our outdoor environment, we propagate the signal outdoors over a distance of up to 90 m, modeling the real-world challenges of QKD in free-space applications and successfully demonstrating BB84 for dimensions 2 and 4.