  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.
