Elitzur-Vaidman bomb tester

In plain terms

The problem, as its inventors stated it, is this. You have a batch of bombs, each with a trigger so sensitive that a single photon sets it off, and some are duds where the trigger is missing and light passes through harmlessly. Your job is to find a live bomb. Testing means shining light on it, and shining light on a live one destroys it, so classically the task is hopeless, because any bomb you've verified you've also detonated.

Quantum mechanics finds a way through, using an interferometer, a device that splits a single photon so it travels along two paths at once and then recombines the paths so they interfere. A dud changes nothing, so the paths interfere as usual and a particular dark output port stays dark. A live bomb spoils the interference merely by being capable of absorbing the photon, and it does so even on runs where nothing was absorbed, so when the dark port clicks you hold a certified live bomb that no photon ever touched.

The basic scheme certifies a quarter of the live bombs and sacrifices half, and cleverer versions push the efficiency as close to perfect as you like.

In CSD

This is one of the corpus's empirical twins, and it's among the sharpest tests an account of quantum mechanics can face, because it forces a definite statement about the path not taken. A story in which the photon simply went one way must explain how an object sitting in the other, untravelled arm changed the outcome, and a story in which nothing was definite must explain what the final record is a record of.

CSD routes its answer through the epistemic regions rather than through a story about paths. In CSD each measurement outcome corresponds to a region of the space of states, and the record is read against those regions, so the presence of the absorber reshapes the region structure the record is read against and the volume bookkeeping does the rest. The point of carrying the case explicitly is that reproducing this experiment is a genuine constraint and not a formality.

Mathematically

An interferometer in which an absorbing object in one arm destroys the interference, so a detection at the normally dark port certifies the object's presence without absorption. The base efficiency is one quarter certified and one half destroyed.

Kwiat-style iterated variants exploit the quantum Zeno effect to drive the certified fraction arbitrarily close to one.

The name

Avshalom Elitzur and Lev Vaidman published the bomb tester in Foundations of Physics in 1993. Vaidman, of Tel Aviv University, is a career foundations physicist, the co- inventor of weak measurements with Aharonov and Albert, and a cheerful, combative Everettian. Elitzur is the wilder card, as he had no doctorate at the time, worked outside the standard track, and was later associated with Bar-Ilan and a private institute, with interests running to time and thermodynamics.

The idea was realised in the laboratory within two years, by Kwiat, Weinfurter, Herzog, Zeilinger and Kasevich at Innsbruck in 1995, using down-converted photons rather than bombs, to general relief.

Related entries

• Mach-Zehnder interferometerSplit a single particle down two paths and recombine them, and where it lands depends on both paths, including the one you can prove it never took.
• Hong-Ou-Mandel effectWhen two identical photons meet at a beam splitter they always leave together and never one through each exit, so their indistinguishability shows up as a hole in the data.
• CollapseIn CSD the quantum state never jumps, and that's a theorem about volume-preserving maps rather than a preference among interpretations.
• What happens during a measurement in CSD?The system stops being isolated, the joint dynamics of system and apparatus carries the underlying state into one outcome region and leaves a record there, and the famous collapse is the updating of a description, so there's one law and no switch.
• De-isolationNothing collapses, as a measured system stops being isolated, and measurement is what de-isolation dynamics looks like from inside.