The bomb you never touch
Can you learn that something is there without a single photon touching it?
Known, reproduced
Yes. A photon that could have hit a bomb and didn't still carries the news that the bomb is there. The simple version (Elitzur and Vaidman) finds a live bomb untouched only 25% of the time. Looping the photon past the bomb many times, with a tiny turn of its polarization (the direction its wave wiggles) each lap, pushes that toward 100%. Real mirrors leak a little light every lap, and that sets the ceiling.
The Zeno loop, live
Each lap turns the photon's polarization by 90°/N toward the bomb's path. The bomb then asks "are you on my path?" with odds sin²(angle). A no snaps the angle back to zero. Each lap the photon can also be lost.
…
left · the photon laps the loop; the arrow is its polarization (up = safe path, sideways = the bomb's path) · right · lines = ((1−L)·cos²(π/2N))N for loss L = 0, 0.1%, 1% per lap · dots = this session's photons.
In plain words
One pass through a two-path interferometer gives a live bomb 50% boom, 25% bright click (no information) and 25% dark click (bomb found, untouched). Firing again after every bright click raises the found share to 1/3.
The loop does better because of the Zeno effect. Each lap's turn is tiny, so the bomb almost always answers "no", and that answer resets the photon. It is the same formula as the watched qubit: 10 laps find the bomb untouched 78% of the time, 100 laps 97.6%. Losses spoil it. At 0.1% loss per lap the best plan is 50 laps and 91%; at 1% it is 16 laps and 73%.
There is a catch. A semi-transparent object also freezes the photon, so the test tells presence, not degree: a mostly see-through object still reads "there".