  We show that an Aharonov-Bohm (AB) ring with asymmetric electron injection
can act as a coherent detector of electron dephasing. The presence of a
dephasing source in one of the two arms of a moderately-to-highly asymmetric
ring changes the response of the system from total reflection to complete
transmission while preserving the coherence of the electrons propagating from
the ring, even for strong dephasing. We interpret this phenomenon as an
implementation of an interaction-free measurement.
