Quantum dense coding without entanglement

Abstract

We describe a quantum cryptography protocol with up to twenty four-dimensional ($\mathcal{D}=4$) states generated by a polarization-, phase- and time-encoding transmitter. This protocol can be experimentally realized with existing technology, drawing from time-encoded and polarization-encoded systems. The protocol is error tolerant and has a quantum bit-rate of 2 per transmission/detection, which when combined with state detection efficiency yields a qubit efficiency of up to 1 or double that of BB84-{\it like} protocols.

New title, 6 pages, 3 figures. Submitted to PRL: 12/23/09. Includes PRL response. Possible it will not be published in PRL; PRA next likely. Nominally related to arXiv:0808.0686, but uniquely different. Tell us ([email protected]) if you find an error.