BB84 protocol using rotationally symmetric states and a Fresnel cone

BB84 is one of the most prominent protocols for quantum key distribution (QKD), and the prototype for all protocols relying on Heisenberg's uncertainty relation to provide security. Photon polarization remains a chosen degree of freedom to implement QKD protocols, as its weak interaction with the environment creates stability during propagation. Generally, polarization-based QKD implementation requires a shared coordinate reference frame between the sender and receiver to maintain the assignment of the mutually unbiased bases. While this is easily realized for stationary systems, it proves challenging for handheld and satellite-to-satellite communication. The use of rotationally symmetric states, including some classes of vector beams, has been proposed to remove the need for a shared reference frame; however, most of these systems are wavelength dependent, thereby limiting available bandwidth. Here, we demonstrate a classical, proof-of-principle BB84 protocol using rotationally symmetric vector beams as the mutually unbiased basis, and a measurement system employing a Fresnel cone. Our results demonstrate that it can yield an error rate of 1.1%. This system is reference frame invariant and can be implemented in a wavelength-independent manner. We expect our technique to be suitable for implementing QKD in scenarios where the sender and receiver's coordinate systems rotate with respect to each other, such as with satellite or handheld communication, while still maintaining large bandwidth capabilities.