ANU physicists develop the most sensitive method to measure the potential energy of the atom

Physicists at the Australian National University have developed the most sensitive method ever used to measure the potential energy of an atom (to less than one hundredth of a decillionth of a joule – or 10-35 joule), and have used to validate one of the most tested theories in physics – quantum electrodynamics (QED).

The research, published this week in Science relies on finding the color of laser light where a helium atom is invisible, and is independent corroboration of previous methods used to test QED, which involved measuring transitions from one atomic energy state to a other.

“This invisibility is only for a specific atom and a specific color of light – so it could not be used to make an invisibility cloak that Harry Potter would use to investigate the dark corners of Hogwarts.” said lead author Bryce Henson, a PhD student at the ANU Research School of Physics.

“But we were able to use to investigate some dark corners of QED theory.”

We were hoping to catch up with QED, as there were some discrepancies between theory and experiments, but it’s been good enough.”

Quantum electrodynamics, or QED, was developed in the late 1940s and describes how light and matter interact, incorporating both quantum mechanics and Einstein’s theory of special relativity in a way that has proven itself for nearly eighty years.

However, hints that the QED theory needed improvement came from discrepancies in proton size measurements, which were mostly resolved by 2019.

Around this time, ANU PhD researcher Bryce Henson noticed small oscillations in a highly sensitive experiment he was conducting on an ultracold cloud of atoms known as the Bose-Einstein condensate.

He measured the frequency of the oscillations with record precision, finding that interactions between atoms and laser light changed the frequency, as the color of the laser varied.

He realized that this effect could be exploited to very accurately determine the precise color at which the atoms did not interact with the laser at all and the oscillation remained unchanged – in other words, effectively became invisible.

With the combination of a very high resolution laser and atoms cooled to 80 billionths of a degree above absolute zero (80 nanokelvin), the team achieved a sensitivity in their energy measurements that was 5 orders less than the energy of the atoms, about 10 – 35 joules, or a temperature difference of about 10-13 degrees Kelvin.

“It’s so small that I can’t think of any phenomenon to compare it to – it’s so far from the end of the scale,” said Mr. Henson.

Thanks to these measurements, the team was able to deduce very precise values ​​for the invisibility color of helium. To compare their results with the theoretical prediction for QED, they turned to Professor Li-Yan Tang from the Chinese Academy of Sciences in Wuhan and Professor Gordon Drake from the University of Windsor in Canada.

Previous calculations using QED had less uncertainty than experiments, but with the new experimental technique improving accuracy by a factor of 20, theorists had to rise to the challenge and improve their calculations.

In this quest they were more than successful – improving their uncertainty to only 1/40th of the last experimental uncertainty and distinguishing the contribution of QED to the atom’s invisibility frequency which was 30 times greater than the uncertainty of experience. The theoretical value was only slightly lower than the experimental value of 1.7 times the experimental uncertainty.

The head of the international collaboration, Professor Ken Baldwin of the ANU Research School of Physics, said improvements to the experiment could help close the gap, but they would also perfect a amazing tool that could shed light on QED and other theories.

“New precision measurement tools often lead to big changes in theoretical understanding along the way,” said Professor Baldwin.


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