= Elitzur-Vaidman bomb tester
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{wiki=Elitzur–Vaidman_bomb_tester}
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.
In 1993, <Avshalom Elitzur> and <Lev Vaidman> asked how to identify a working bomb whose sensor would be triggered by even one <photon>. Their theoretical proposal places the sensor in one arm of a <Mach-Zehnder interferometer>, adjusted so that <destructive interference> keeps one detector dark when both arms are unobstructed. A click there can identify a working bomb while leaving it unexploded. The motivation is to obtain information about a fragile object without destroying it; the bomb makes that operational claim explicit without committing to an interpretation of <quantum mechanics>.<Quantum mechanical interaction-free measurements by Avshalom C. Elitzur and Lev Vaidman>[1993 author preprint, §4 “How to test a bomb without exploding it?”, pp. 8–9; interferometer setup in §1, p. 3]{ref}
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