Elitzur–Vaidman Bomb Tester

Bomb Type

Zeno Stages

Number of interferometer stages N 12

Ready — press Fire.

Predicted Probabilities

P(explode)

50.0%

P(IFM success)

25.0%

Per-stage angle θ

7.5°

P(normal detector)

25.0%

Trial Statistics

Exploded

IFM success

Normal det.

Trials run

Empirical success rate

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How it works

A photon enters a chain of N very weak beam splitters that would rotate its state from the "safe" rail to the "test" rail by a total angle of 90° if nothing disturbs it — each stage contributes a small rotation θ = π/(2N).

θ = π / (2N)
No bomb / dud bomb:  photon always ends up fully rotated
                      into the TEST rail  →  normal detector.

Live bomb present:  each stage is a which-path measurement.
 P(no explosion after N stages) = cos^(2N)(θ)
 P(explode)                     = 1 − cos^(2N)(θ)
 A surviving photon is projected back to the SAFE rail at
 every stage (quantum Zeno effect), so if it survives all N
 stages it is found at the IFM detector — proof a bomb sits
 in the device, without a single photon ever touching it.

Bomb type — a live bomb absorbs any photon that reaches it (which-path information ⇒ collapse); a dud is transparent and reveals nothing, so it behaves exactly like "no bomb".
N stages — more, weaker stages exploit the quantum Zeno effect: the explosion probability falls toward 0% while the interaction-free success probability climbs toward 100% as N → ∞.
Fire one photon — plays a single trial through the ring, stage by stage, with the real outcome probabilities above.
Run 300 trials — samples many trials instantly to check the empirical rates against the analytic prediction.

This is the Elitzur–Vaidman bomb tester (1993), extended to high efficiency by Kwiat, Weinfurter, Herzog, Zeilinger & Kasevich (1995) — the first scheme to show that quantum mechanics lets you learn the *existence* of an object through counterfactual, interaction-free measurement.

Photon (safe rail) Leak toward bomb Live bomb

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Elitzur–Vaidman Bomb Tester — Interaction-Free Quantum Measurement