Underwater quantum communication over a 30-meter flume tank

Abstract: Underwater quantum communication has recently been explored using polarization and orbital angular momentum. Here, we show that spatially structured modes, e.g., a coherent superposition of beams carrying both polarization and orbital angular momentum, can also be used for underwater quantum cryptography. We also use the polarization degree of freedom for quantum communication in an underwater channel having various lengths, up to $30$ meters. The underwater channel proves to be a difficult environment for establishing quantum communication as underwater optical turbulence results in significant beam wandering and distortions. However, the errors associated to the turbulence do not result in error rates above the threshold for establishing a positive key in a quantum communication link with both the polarization and spatially structured photons. The impact of the underwater channel on the spatially structured modes is also investigated at different distances using polarization tomography.