  We construct a device that can unambiguously discriminate between two unknown
quantum states. The unknown states are provided as inputs, or programs, for the
program registers and a third system, which is guaranteed to be prepared in one
of the states stored in the program registers, is fed into the data register of
the device. The device will then, with some probability of success, tell us
whether the unknown state in the data register matches the state stored in the
first or the second program register. We show that the optimal device, i. e.
the one that maximizes the probability of success, is universal. It does not
depend on the actual unknown states that we wish to discriminate.
