  We present a method for intrusion detection which is based on the
Mach-Zehnder interference effect. This device provides monitored surveillance
by continuously measuring the intensity of light collected by a pair of
photodetectors. We find that our protocol allows for the detection of intrusion
attempts which employ path- redirection and/or intercept-resend techniques.
Expectation values for the registered output flux are provided for normal and
interrupted operation.
