  We propose a quantum circuit to discriminate between two arbitrary quantum
evolution operators. It permits to test the equality of two quantum operators
and to estimate a fidelity measure of them. The relation of the proposal to the
SWAP test for discriminating two quantum states is analyzed. We also discuss
potential applications for the discrimination of quantum communication channels
and possible laboratory implementations with light along the same lines of
recent experimental realizations of quantum superpositions of causal orders
exploiting the different degrees of freedom of photons. We also discuss
hardware efficient realizations for noisy intermediate scale quantum computers.
