Demonstration of a 10 Mbit/s quantum communication link by encoding data on two Laguerre–Gaussian modes with different radial indices

We experimentally demonstrate a 10 Mbit/s free-space quantum communication link using data encoding on orthogonal Laguerre–Gaussian (LG) modes with the same azimuthal index but different radial indices. Data encoding on two LG_lp modes (i.e., for l 0, we encode [“0”, “1”] as [p 0, p 1], and for l 1, we encode [“0”, “1”] as [p 0, p 1]) is demonstrated by employing directly modulated laser diodes and helical phase holograms. The quantum symbol error rate (QSER) of <5% is achieved at an encoding rate of 10 Mbit/s. Moreover, the influence of the circle radius (R) of the receiver phase pattern on registered photon rates and QSERs is investigated. Our results show that a receiver phase pattern whose R does not match the beam size of the LG modes would induce higher cross talk between the two encoded quantum branches.