Degrees of freedom (mechanics) 1 Efficiency 1 Key bits Key distribution 1 Key-distribution protocols 1 Particle beams 1 Photons 1 Polarization 1 Polarization degree 1 Polarization modes 1 Private key 1 Quantum communication 1 Quantum efficiency 1 Semiquantum key distribution 1 Semiquantum key distribution (SQKD) 1 Single photon 1 Single photons 1 Spatial distribution 1 Spatial modes 1 Spatial-mode degree 1