A new version of the famous double-slit experiment has allowed physicists in Israel to measure a phenomenon that is bizarre even by the counterintuitive standards of quantum mechanics. By placing a double-slit experiment along one path of a larger double-slit experiment, the researchers have shown that photons traverse a section of the apparatus that they neither enter nor exit. The effect, the team argues, is best understood by invoking a little-used interpretation of quantum mechanics that was first proposed in 1955.
Perhaps the simplest and starkest demonstration of wave–particle duality is the famous double-slit experiment. Particles such as photons or electrons that are emitted discretely behave as waves when they pass through two slits and build-up an interference pattern when detected individually on a screen.
In this latest version of the experiment, Lev Vaidman and colleagues at Tel-Aviv University used Mach–Zehnder interferometers as double slits and photons as particles. The optical interferometer uses a beamsplitter to divide the photon beam into two separate paths that are then recombined and sent to a detector. A difference in the lengths of the two paths dictates how the beams interfere when recombined, which affects the intensity measured by the detector.
Three possible paths
In the Tel Aviv experiment, an inner Mach–Zehnder interferometer is placed in one path of an outer interferometer so that the recombined beam continues its journey through the outer device and on to a detector (see figure below). This means that a photon has three possible paths from source to detector. The goal of the experiment is to find out which paths are taken by at least some photons arriving at the detector. This is called a weak measurement, and is consistent with the laws of quantum mechanics because it does not involve measuring the path of any specific photon.
To make their measurements, the researchers set all the mirrors in the interferometer vibrating slightly, each at a different frequency. As a mirror vibrates, it alters the pathlength of any light reflecting from that mirror. This alters the phase difference when the beam is recombined, changing the intensity at the detector. As every mirror is vibrating at a unique frequency, oscillations in the detected intensity at a particular frequency indicated that photons have touched a specific mirror.
The researchers arranged the two pathlengths through the inner interferometer so that the two paths interfered destructively when they recombined. Therefore, no light could leave the inner interferometer. One might expect, therefore, that the only oscillation in the detected intensity would come from the mirror bypassing the inner interferometer, but this was not what the researchers found.
Tim Wogan is a science writer based in the UK