We describe a four-dimensional (D = 4) single-photon quantum cryptography protocol with up to twenty (D×(22 +1)) possible states generated by a polarization-, phase- and time-encoding transmitter. This protocol can be experimentally realized with existing technology, drawing from time- and polarization-encoded systems. The protocol is error tolerant and has a maximum raw bit rate of two raw bits per detection, which when combined with state detection efficiency yields a qubit rate of up to one per transmission under ideal assumptions, or up to twice the raw bit rate of two-dimensional protocols such as the well-known BB84 protocol.
['W. T. Buttler', 'Steven K. Lamoreaux', 'J. R. Torgerson']
Practical four-dimensional quantum key distribution without entanglement