The build up of an interference pattern - the classic signature of wave-like behaviour - in an experiment performed with molecules is reported online this week in Nature Nanotechnology. The ability of particles to behave as waves is one of the defining features of quantum mechanics - the theory that describes the behaviour of matter on the smallest length scales - and the molecules in this work have typical wavelengths of a few picometres.
The observation of an electron interference pattern was once described as the “most beautiful experiment in physics”. The experiment involved recording the slow build up of an interference pattern as individual electrons hit a detection screen after passing through two narrow slits. More recently, Markus Arndt and co-workers observed such wave-like behaviour in experiments with molecules containing more than 400 atoms. Now Arndt and co-workers have combined these two feats to record a movie showing the build up of an interference pattern as individual molecules (which contain either 58 or 114 atoms) are detected after passing through slits.
A classic way to demonstrate wave-like behaviour is to send a beam of light, which is a wave, through a pair of narrow slits and measure how the intensity of the light hitting a screen behind the slits changes with position. Peaks are seen in the intensity wherever a maximum in the wave that has passed through one slit coincides with a maximum in the wave that has passed through the other slit, and troughs are observed wherever two minima coincide with each other. Such a pattern of peaks and troughs is known as an interference pattern. If this experiment is repeated with a beam of ordinary particles we will see two peaks, each formed by the particles that have passed through a particular slit. However, if we use a beam of quantum particles we will see an interference pattern with multiple peaks, just as we do for light. As Bum Suk Zhao and Wieland Schollkopf write in an accompanying News and Views article, the latest experiments “should provide new insights into the differences between the quantum and classical worlds”.