Young’s experiment in a hydrogen molecule

An international investigation involving the participation of the Consejo Superior de Investigaciones Científicas (CSIC) has reproduced the experiment of Thomas Young in a molecule of hydrogen, the smallest molecular system that exists. In 1803 the English scientist tested a pattern of interferences in light from a distant source, on passing through a “double slit” and thus being refracted.

This finding confirmed the theory that light had wave motion properties. The authors of this current research, which appears in the latest issue of the journal Science, uses electrons instead of light and the nuclei of the hydrogen molecule as emitting slits.

CSIC researcher Ricardo Díez, Vicedirector of the Centre for Materials Physics (a mixed body of the CSIC and the University of the Basque Country in Donostia-San Sebastián and co-author of the article, explains their experiment: “These interference patterns are the same as those produced, on a large scale, when sunlight passes through Persian blinds, throwing shadow patterns and, as it were, games, on the walls. This phenomenon is due to the fact that (light) particles, as with electrons, can also have wave motion behaviour”.

At much smaller sizes, atomic planes can create interferences in the transmission of X rays, thus providing information about the internal structure of materials. This is the fundamental basis of the experimental techniques such as X ray diffraction, thanks to which the DNA double helix structure was discovered. Ricardo Díez explains, “The Laws that predict, for example, the trajectory of a car at a certain speed are not those that govern the behaviour of atomic-sized particles. On a nanometric scale sizes are measured in units a thousand million times smaller than a metre, and the behaviour of objects at this scale can prove to be surprising, almost magical even!”

The experiment

The researchers reproduced Young’s experiment in the smallest system existing – a molecule of hydrogen -, which consists of two protons and two electrons. The research team used light generated by the large synchrotron accelerator at the Lawrence Berkeley National Laboratory (USA), to extract the two electrons from the molecule of hydrogen. The two protons carry out the role of the two electron-emitting apertures, separated by an extremely small distance – ten thousand millionths of a metre. On its journey to the detector, where they are collected, each one of the electrons shows an interference pattern that suggests wave nature rather than particle motion, and as if emission had taken place from the two points at the same time.

The interference pattern of each one of the two electrons extracted from the molecule is conditioned by the presence and the velocity of the other: the greater the difference in their speeds, the less the interaction between them and the more visible the interference patterns. Under these conditions, the system is more of a quantum nature. “The analysis of the patterns as a function of velocity enables the investigation of the subtle mechanisms of the transition between classical physics and quantum physics. It is necessary to understand the quantum relationship between a small number of electrons, such as those of hydrogen, as it is the basis of concepts as sophisticated as quantum cryptography or of the future development of quantum computation”, concluded the CSIC researcher.

The study was led by University of Frankfurt researcher Reinhard Dörner and involved, moreover, the participation of German North American and Russian scientists.