An international team of physicists has proposed a new device that could detect the presence of waves or particles while barely disturbing them. Called a “Schrödinger’s hat”, the device has not yet been built in the lab but the team believes that it could someday be used as a new type of sensor for quantum-information systems.
In the microscopic world of quantum mechanics, direct observation of the property of a particle – the position of an electron, for example – causes the collapse of the particle’s wavefunction. The result is that the particle that you set out to measure has been changed in a significant way.
In the early 1990s, physicists Avshalom Elitzur and Lev Vaidman at Tel Aviv University in Israel pointed out that it is not always necessary to observe particles directly to learn something of their nature. The researchers imagined a pile of bombs, each of which is designed to be triggered by the absorption of a single photon. Some of the bombs are duds; through these, photons pass unimpeded. One could check whether a bomb is working by firing a photon at it but, if the bomb were indeed to be working, it would be destroyed in the process. Would there be a way to weed out some of the working bombs without destroying them?
The answer is yes, say Elitzur and Vaidman. They considered an interferometer: a device through which a photon’s path is split into two arms, only to recombine at a set of detectors some distance away. To test a bomb, it would have to be placed in one arm of the interferometer. A dud bomb would have no effect on the photon, and the photon would pass through both arms, generating an interference pattern at the detectors. A working bomb, on the other hand, would force the photon to “choose” through which arm it passes. If it took the bomb’s arm, the bomb would, regrettably, be triggered. If the photon took the empty arm, it would reach the detectors unimpeded – but since the other arm was blocked, there would be no interference pattern. This lack of an interference pattern would reveal the existence of the working bomb without having triggered it.