Quantum tunnelling in water confirmed

A quantum theory first proposed nearly 90 years ago has at last been confirmed, potentially galvanising new approaches in fields as diverse as medical biosensing and solar energy storage.

University of Sydney researchers David McKenzie and Enyi Guo report in the journal Proceedings of the Royal Society that they have successfully demonstrated quantum tunnelling in water – a phenomenon first predicted by British theoretical physicist Ronald Gurney in 1931.

Quantum tunnelling is one of the weirder products of quantum mechanics, and relies on the wave-particle duality of subatomic particles. 

The theory describes how in certain circumstances a particle will defeat the constraints of classical physics when confronted by a barrier. Classical equations will show that the particle does not have enough energy to physically overcome the obstacle, but wave-particle duality allows it to tunnel straight through it.

The ability of particles to tunnel is now well established. It is critical to nuclear fusion, and is the basis of the scanning tunnelling microscope (STM). However, despite predictions, until now it had never been observed in water.

To explore Gurney’s theory, McKenzie and Guo placed gold electrodes into pure water and ran a low voltage current between them. 

They then set about measuring the contribution to the steady current arising from two processes: the neutralisation of ions absorbed onto the surface of the electrodes, and the tunnelling of electrons between the electrodes and ions still in solution.

They determined that electrons tunnelling to and from ions near the electrodes accounted for the greater part of the current. They also found that Gurney’s calculations were a bit wide of the mark when the actual values became known, but confirmed that later refinements made by Canadian-born chemist and Nobel laureate Rudolph Marcus were on the money.

For non-theoretical physicists, the discovery might seem somewhat recondite, but McKenzie points out that his and Guo’s findings have significant ramifications for some very big industries.

“This lays the basis for new and faster methods to detect biomedical impurities in water, with potentially important implications for biosensing techniques,” he says.

“A better understanding of electrolysis is becoming more important for applications in alternative energies in what is sometimes called the ‘hydrogen economy’.”