Invisible helium atoms offer an extremely sensitive test of fundamental theory

A Bose-Einstein condensate machine using metastable helium. Photo credit: Tracey Nearmy, Australian National University

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

The research, published this week in Science is based on finding the color of the laser light, where a helium atom is invisible and is an independent confirmation of previous methods of testing QED, which measured transitions from one atomic energy state to another.

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

“But we were able to examine some dark corners of the QED theory.”

“We were hoping to catch QED because there used to be some discrepancies between theory and experiments, but it passed with a pretty good grade.”

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

However, evidence that the QED theory needed improvement came from discrepancies in measurements of the size of the proton, most of which were resolved in 2019.

Around this time ANU Ph.D. Scholar Bryce Henson noticed small oscillations in a very sensitive experiment he performed on an ultracold atomic cloud known as the Bose-Einstein condensate.

He measured the frequency of the vibrations record precisionFinding that interactions between the atoms and the laser light changed the frequency as the laser color was varied.

He realized that this effect could be used to very precisely determine the exact color at which the atoms did not interact with the laser at all and the vibration remained unchanged, in other words became practically invisible.

With the combination of an extremely high-resolution laser and atoms cooled to 80 billionths of a degree above absolute zero (80 nanokelvins), the team achieved a sensitivity in their energy measurements that was 5 orders of magnitude lower than the energy of the atoms, about 10–35 Joules or a temperature difference of about 10-13 of one degree Kelvin.

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

With these measurements, the team was able to derive very accurate 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 the experiments, but with the new experimental technique improving the accuracy by a factor of 20, theorists had to rise to the challenge and improve their calculations.

In this search they were more than successful – they improved their uncertainty to just 1/40th of the last experimental uncertainty and singled out the QED contribution to the atom’s invisibility frequency, which was 30 times greater than the experimental uncertainty. The theoretical value was slightly lower than the experimental value, only 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 resolve the discrepancy, but would also refine an extraordinary tool that could shed light on QED and other theories.

‘New tools for precision measurements often lead to major shifts in theoretical understanding,’ said Professor Baldwin.

JILA atomic clocks measure Einstein’s general theory of relativity in the millimeter range

More information:
BM Henson et al, Measuring a helium tuning frequency: an independent test of quantum electrodynamics, Science (2022). DOI: 10.1126/science.abk2502

Citation: Invisible Helium Atoms Offer an Exquisitely Sensitive Test of Fundamental Theory (2022 April 8) Retrieved April 8, 2022 from

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