On interaction-free measurement

Abstract: This manuscript is inspired by the paper [2]. In the paper, they investigate a method to detect existence of an object with arbitrarily small interaction. Below, we sketch their protocol to motivate the present manuscript. The object of their protocol is to detect whether the given blackbox interact with input states or not, with negligible distortion of the blackbox, and high detection probability. In this paper, we do two things. First, we prove the above mentioned protocol is optimal in a certain setting. The main tool here is adversary method, a classical method in query complexity. Second, we present a protocol to detect unitary operations with negligible error and no distortion of the input at all.