  We introduce a new quantum key distribution protocol that uses d-level
quantum systems to encode an alphabet with c letters. It has the property that
the error rate introduced by an intercept-and-resend attack tends to one as the
numbers c and d increase. In dimension d=2, when the legitimate parties use a
complete set of three mutually unbiased bases, the protocol achieves a quantum
bit error rate of 57.1%. This represents a significant improvement over the 25%
quantum bit error rate achieved in the BB84 protocol or 33% in the six-state
protocol.
