Students help overturn conventional expectation in quantum physics

The final-year projects of two Electrical and Computer Systems engineering students, Mehdi Pourzand and Chu Rui Jian, at Monash Malaysia led to a surprising discovery in quantum physics.

To study the motion of a microscopic system in the non-relativistic regime, the standard scientific practice is to use non-relativistic quantum theory, instead of the mathematically and computationally more complicated relativistic version. This is because the former is conventionally thought to always yield similar results to the latter.

In the paper published recently in the journal ‘Results in Physics, the duo together with their supervisor, Associate Professor Lan Boon Leong, proved that the conventional expectation is not true in general.

The surprising conclusion in the paper is based on a key mathematical insight. The corresponding terms in the non-relativistic and relativistic equations for the time-dependent wave function (a fundamental quantity in quantum theory) differ by a phase factor, where the phase depends on the small difference between the non-relativistic and relativistic energies. However, because the phases grow with time, the two predictions will eventually disagree.

Their numerical results showed the breakdown of agreement could happen quickly. The results also showed the different predictions could be tested experimentally for the simplest atom ― hydrogen.

“Our finding implies that the relativistic theory must be used instead to correctly predict the motion after the agreement between the two theories breaks down. This paradigm shift opens a new avenue of research in a wide range of fields from atomic to molecular, chemical and condensed-matter physics, which could lead to new understandings and discoveries,” said Lan, who is the Head of the Electrical and Computer Systems Engineering discipline.

“Our finding also suggests some of the numerous studies, which utilised non-relativistic quantum theory, in the past are not entirely correct. For example, the puzzling discrepancies between the non-relativistic quantum predictions and experimental measurements for the simplest chemical reaction involving the collision between hydrogen atom and deuterium molecule might be resolved by using relativistic quantum theory,” he added.

Both Pourzand and Chu, who have since graduated, shared that even though the project took them out of their comfort zone, they enjoyed it tremendously. It has also ignited their passion for research.

“It is uncommon to have the opportunity to engage in a serious scientific research at the undergraduate level. I’m very happy that our group effort led to an important discovery,” Pourzand said.

“I have learned a lot about how to conduct research. But the most valuable thing I have learned is that we should always question conventional wisdom,” said Chu.

Chu is pursuing his PhD in signal and image processing in France, and Pourzand will be working on his Masters in experimental atomic and molecular physics in Canada.

To read the full paper, it can be accessed here: https://doi.org/10.1016/j.rinp.2018.11.050