  We discuss a scheme in which sequential state-discrimination measurements are
performed on qudits to determine the quantum state in which they were initially
prepared. The qudits belong to a set of nonorthogonal quantum states and hence
cannot be distinguished with certainty. Unambiguous state discrimination allows
error-free measurements at the expense of occasionally failing to give a
conclusive answer about the state of the qudit. Qudits have the potential to
carry more information per transmission than qubits. We considered the
situation in which Alice sends one of N qudits, where the dimension of the
qudits is also N. We look at two cases, one in which the states all have the
same overlap and one in which the qudits are divided into two sets, with qudits
in different sets having different overlaps. We also study the robustness of
our scheme against a simple eavesdropping attack and found that by using qudits
rather than qubits, there is a greater probability that an eavesdropper will
introduce errors and be detected.

Quantum Physics