Quantum wonders: The Elitzur-Vaidman bomb-tester

 作者:杞楞     |      日期:2019-03-14 08:11:00
A BOMB triggered by a single photon of light is a scary thought. If such a thing existed in the classical world, you would never even be aware of it. Any photon entering your eye to tell you about it would already have set off the bomb, blowing you to kingdom come. With quantum physics, you stand a better chance. According to a scheme proposed by the Israeli physicists Avshalom Elitzur and Lev Vaidman in 1993, you can use quantum trickery to detect a light-triggered bomb with light – and stay safe a guaranteed 25 per cent of the time (Foundations of Physics, vol 23, p 987). The secret is a device called an interferometer. It exploits the quantumly weird fact that, given two paths to go down, a photon will take both at once. We know this because, at the far end of the device, where the two paths cross once again, a wave-like interference pattern is produced (see “Quantum wonders: Corpuscles and buckyballs”). To visualise what is going on, think of a photon entering the interferometer and taking one path while a ghostly copy of itself goes down the other. In Elitzur and Vaidman’s thought experiment, half the time there is a photon-triggered bomb blocking one path (see diagram). Only the real photon can trigger the bomb, so if it is the ghostly copy that gets blocked by the bomb, there is no explosion – and nor is there an interference pattern at the other end. In other words, we have “seen” the bomb without triggering it. “The computer retains a semblance of classical decency: to deliver a sensible answer,