The Elitzur–Vaidman bomb-tester is a quantum mechanics thought experiment that uses interaction-free measurements to verify that a bomb is functional without having to detonate it. It was conceived in 1993 by Avshalom Elitzur and Lev Vaidman. Since their publication, real-world experiments have confirmed that their theoretical method works as predicted.
The bomb tester takes advantage of two characteristics of elementary particles, such as photons or electrons: nonlocality and wave–particle duality. By placing the particle in a quantum superposition, it is possible for the experiment to verify that the bomb works without triggering its detonation, although there is still a 50% chance that the bomb will detonate in the effort.
Background
The bomb test is an interaction-free measurement. The idea of getting information about an object without interacting with it is not a new one. For example, there are two boxes, one of which contains something, the other of which contains nothing. If you open one box and see nothing, you know that the other contains something, without ever opening it.
This experiment has its roots in the double-slit experiment and other, more complex concepts which inspired it, including Schrödinger's cat, and Wheeler's delayed-choice experiment. The behavior of elementary particles is very different from what we experience in our macroscopic world. Their observed behavior can be that of a wave or of a particle (see wave–particle duality), their wave-like behavior implies what is called "superposition". In this state, some properties of the particle, for example, its location, are not definite. While in a superposition, any and all possibilities are equally real. So, if the particle could feasibly exist in more than one location, in certain senses that are experimentally useful, it exists in all of them simultaneously. The particle's wave can later be "collapsed" by observing it, at which time its location (or other measured property) at the moment of observation is definite. Information can then be gleaned not only about the actual state of the particle, but also other states or locations in which it "existed" before the collapse. This gleaning of information is possible even if the particle was never factually in any of the particular states or locations that are of interest.
How it works
Consider a collection of light-sensitive bombs, of which some are duds. When their triggers detect any light, even a single photon, the light is absorbed and the bomb explodes. The triggers on the dud bombs have no sensor, so any light incident on the bomb will not be absorbed and will instead pass straight through. The dud bomb will not detect any photon and will not detonate. Is it possible to determine which bombs are functional and which are duds without detonating all of the live ones?
Components
A light-sensitive bomb: it is not known whether it is live or a dud. A photon emitter: it produces a single photon for the purposes of the experiment. A photon: after being emitted, it travels through the box below. A "box" which contains: An initial half-silvered mirror: the photon enters the box when it encounters this "beam splitter". The photon will either pass through the mirror and travel along the "lower path" inside the box, or be reflected at a 90-degree angle and travel along the box's "upper path". The bomb in question: the bomb is placed inside the box beforehand on the "lower path". If the bomb is live and comes into contact with a photon, it will detonate and destroy itself and the photon. If, however, the bomb is a dud, the photon passes it by and continues on its way along the lower path. A pair of ordinary mirrors: one mirror is located on each beam path. They are positioned to redirect the photon so that the two paths intersect one another at the same position as the second beam splitter. A second beam splitter: identical to the initial one. This beam splitter is positioned opposite the first, at the intersection between the lower path and upper path (after they have been redirected by the ordinary mirrors), at the exit of the box. A pair of photon detectors: located outside the box, they are aligned with the second beam-splitter. The photon can be detected at either or neither, but never both.