  The best qubit one-way quantum key distribution (QKD) protocol can tolerate
up to 14.1% in the error rate. It has been shown how this rate can be increased
by using larger quantum systems. The polarization state of a biphoton can
encode a three level quantum system - a qutrit. The realization of a QKD system
with biphotons encounters several problems in generating, manipulating and
detecting such photon states. We define those limitations and find within them
a few protocols that perform almost as well as the ideal qutrit protocol. One
advantage is that these protocols can be implemented with minor modifications
into existing single photon systems. The security of one protocol is proved for
the most general coherent attacks and the largest acceptable error rate for
this protocol is found to be around 17.7%. This is the first time, to the best
of our knowledge, that the security of qutrit QKD protocols is rigorously
analyzed against general attacks.
