  Non-reciprocal devices are of increasing interest in quantum information
technologies. This paper examines whether the presence of a non-reciprocal
device in an optical channel is detectable by the communicating parties. We
find that a non-reciprocal device such as a Faraday Rotator results in a
measurable geometric phase for the light propagating through the channel and
that, when using entangled photon pairs, the resulting phase is non-local and
robust against malicious manipulation.
