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We study the number of measurements required for quantum process tomography under prior information, such as a promise that the unknown channel is unitary. We introduce the notion of an interactive observable and we show that any unitary channel acting on a d-level quantum system can be uniquely identified among all other channels (unitary or otherwise) with only O(d^2) interactive observables, as opposed to the O(d^4) required for tomography of arbitrary channels. This result generalizes, sodoi:10.1063/1.4867625 fatcat:sufuuxiy6vg4pl5bfpftltz4p4