In Physics, the Elitzur-Vaidman bomb-testing problem is a thought experiment in quantum mechanics, first proposed by Avshalom Elitzur and Lev Vaidman in 1993. An actual bomb-tester was constructed and successfully tested by Anton Zeilinger, Paul Kwiat, Harald Weinfurter, and Thomas Herzog in 1994. It employs a Mach-Zehnder interferometer for ascertaining whether a measurement has taken place.

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Start with a Mach-Zehnder interferometer and a light source which emits single photons. When a photon emitted by the light source reaches a half-silvered plane mirror, it has equal chances of passing through or reflecting. On one path, place a bomb (B) for the photon to encounter. If the bomb is working, then the photon is absorbed and triggers the bomb. If the bomb is non-functional, the photon will pass through the dud bomb unaffected.

When a photon's state is non-deterministically altered, such as interacting with a half-silvered mirror where it non-deterministically passes through or is reflected, the photon undergoes quantum superposition, whereby it takes on all possible states and can interact with itself. This phenomenon continues until an observer interacts with it, causing the wave function to collapse and returning the photon to a deterministic state.

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[There] are only three observable results:

1. The bomb explodes.
2. The bomb does not explode and only detector (C) detects the photon. The bomb must be usable.
3. The bomb does not explode and only detector (D) detects the photon. It is possible that the bomb is usable or that it is a dud.

In the case of the third observation, the experiment may be repeated to see if the bomb will explode or if detector (C) will detect a photon. On average, this will identify all of the dud bombs, explode two thirds of the usable bombs, and identify one third of the usable bombs without detonating them.

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