The Elitzur–Vaidman bomb tester is a thought experiment in quantum mechanics, proposed by physicists Avshalom C. Elitzur and Lev Vaidman in 1993. It illustrates the concept of using quantum superposition and interference to perform measurements that can detect the presence of a potentially dangerous object (like a bomb) without detonating it.
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A photon in a quantum superposition of a safe path and an absorbing path can sometimes certify a live absorber without being absorbed. Recombination is chosen so that one output is impossible if the absorber is absent. A detection at that output is then an unambiguous successful test. The measurement back-action of a surviving live-absorber trial changes the interference pattern.