The rate of quantum tunneling between hydrogen molecules and deuterium ions was measured experimentally, confirming theoretical calculations. Important discoveries for quantum physics.
The rate at which the rare but crucial quantum phenomenon known as tunneling occurs has been measured experimentally for the first time and found to match theoretical calculations. Theoretical estimates in this area were considered very uncertain, so confirmation in a specific case allows for greater confidence in calculating the frequency of other tunneling events.
Quantum tunneling is one of many phenomena in which subatomic particles behave in ways that classical physics would consider impossible. In this case, an object trapped in a way that would classically require some energy to escape leaves the trap, despite having less than that amount of energy. It is a consequence of, and evidence of, the dual wave/particle nature of objects like electrons – a pure particle could not escape, but a wave occasionally can. Phenomena such as alpha decay of atomic nuclei depend on quantum tunneling to occur.
Tunneling is essential to quantum physics, and calculations based on simple examples are present in university courses. Real-world examples are considerably more complex; knowing that tunneling will occasionally occur in a specific situation and knowing how often are very different things. In a new paper, a team from the University of Innsbruck provides the first measurement of the reaction between a hydrogen molecule and a deuterium anion, finding that it is the slowest reaction ever observed involving charged particles.
Although there is no solid wall keeping deuterium anions and hydrogen molecules apart, physicists imagine the energy barrier as a physical wall, which quantum tunneling occasionally allows protons to penetrate.
University of Innsbruck/Harald Ritsch
The reaction (H2 + D- → H- + HD) involves an exchange between a molecule of two hydrogen atoms – protons without neutrons – and an atom composed of a proton and a neutron orbited by two electrons. After tunneling has occurred, one of the molecule’s components has a neutron, while the unconnected, still negatively charged atom has no neutrons. Although it appears that a neutron has been transferred, the reaction is considered to represent an exchange of protons.
Since hydrogen still makes up the majority of the universe, events like this that don’t require heavier elements occur very frequently on cosmic scales, even though the odds of a specific encounter between hydrogen and deuterium are low. Furthermore, if we are to have any hope of modeling more complex tunneling events, we need to anchor our estimates with measurements from simpler examples like this one.
The Innsbruck team tested the occurrence rate experimentally by filling a trap with a mix of deuterium ions cooled to 10 K (-263°C/-441°F) (cooled by collisions to 15 K) and hydrogen gas. At these temperatures the transfer is classically impossible, but the presence of negatively charged hydrogen ions after 15 minutes indicated that it had happened, although not often.
The rate is measured in cubic centimeters per second, giving a value of 5.2 × 10-20 cubic centimeters per second, with a margin of error of about a third, which probably wouldn’t mean much to anyone except a quantum physicist.
However, this results in a transfer that occurs once every hundred billion times a deuterium anion collides with a hydrogen molecule. This might seem too rare to worry about, but even a small portion of gas contains many billions of molecules. Add enough deuterium and the number of collisions becomes immense.
Measuring the rate “requires an experiment that allows very precise measurements and can still be described quantum mechanically,” said senior author Professor Roland Wester in a statement. The idea for the experiment came to Wester 15 years ago, but tunneling is so rare that it took considerable effort to build an experiment in which it could be measured.
The study is published on Nature.
An earlier version of this article was published in .