  We present here a quantum tripwire, which is a quantum optical interrogation
technique capable of detecting an intrusion with very low probability of the
tripwire being revealed to the intruder. Our scheme combines interaction-free
measurement with the quantum Zeno effect in order to interrogate the presence
of the intruder without interaction. The tripwire exploits a curious nonlinear
behaviour of the quantum Zeno effect we discovered, which occurs in a lossy
system. We also employ a statistical hypothesis testing protocol, allowing us
to calculate a confidence level of interaction-free measurement after a given
number of trials. As a result, our quantum intruder alert system is robust
against photon loss and dephasing under realistic atmospheric conditions and
its design minimizes the probabilities of false positives and false negatives
as well as the probability of becoming visible to the intruder.
