Invisible helium atoms provide exquisitely sensitive test of fundamental theory
Date:
April 8, 2022
Source:
Australian National University
Summary:
Physicists have developed the most sensitive method ever for
measuring the potential energy of an atom (within a 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).
FULL STORY ========================================================================== Physicists at the Australian National University have developed the most sensitive method ever for measuring the potential energy of an atom
(within a 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 relies on finding the colour
of laser light where a helium atom is invisible, and is an independent corroboration of previous methods used to test QED, which have involved measuring transitions from one atomic energy state to another.
"This invisibility is only for a specific atom and a specific colour of
light - - so it couldn't be used to make an invisibility cloak that Harry Potter would use to investigate dark corners at Hogwarts," said lead
author, Bryce Henson, a PhD student at ANU Research School of Physics.
"But we were able to use to investigate some dark corners of QED theory."
"We were hoping to catch QED out, because there have been some previous discrepancies between theory and experiments, but it passed with a pretty
good mark." 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 remained successful for nearly eighty years.
========================================================================== However, hints that QED theory needed some improvement came from
discrepancies in measurements of the size of the proton, which were
mostly resolved in 2019.
Around this time ANU PhD Scholar Bryce Henson noticed small oscillations
in a very sensitive experiment he was conducting on an ultracold cloud
of atoms known as a Bose-Einstein condensate.
He measured the frequency of the oscillations with record precision,
finding that interactions between the atoms and the laser light changed
the frequency, as the laser colour was varied.
He realised this effect could be harnessed to very accurately determining
the precise colour at which the atoms did not interact at all with the
laser and the oscillation remained unchanged -- in other words effectively becoming invisible.
With the combination of an extremely 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 of magnitude less than energy of the atoms, around 10- 35 joules,
or a temperature difference of about 10-13 of a degree kelvin.
========================================================================== "That's so small that I can't think of any phenomenon to compare it to --
it's so far off the end of the scale," Mr Henson said.
With these measurements the team were able to deduce very precise values
for the invisibility colour of helium. To compare their results with theoretical prediction for QED, they turned to Professor Li-Yan Tang
from the Chinese Academy of the 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, the theoreticians had to rise to the challenge and improve
their calculations.
In this quest they were more than successful -- improving their
uncertainty to a mere 1/40th of the latest experimental uncertainty, and singling out the QED contribution to the atom's invisibility frequency
which was 30 times larger than the experiment's uncertainty. The
theoretical value was only slightly lower than the experimental value
by 1.7 times the experimental uncertainty.
Leader of the international collaboration, Professor Ken Baldwin from the
ANU Research School of Physics, said that improvements to the experiment
might help resolve the discrepancy, but would also hone an extraordinary
tool that could illuminate QED and other theories.
"New tools for precision measurements often drive big changes in
theoretical understanding down the track," Professor Baldwin said.
========================================================================== Story Source: Materials provided by Australian_National_University. Note: Content may be edited for style and length.
========================================================================== Journal Reference:
1. B. M. Henson, J. A. Ross, K. F. Thomas, C. N. Kuhn, D. K. Shin,
S. S.
Hodgman, Yong-Hui Zhang, Li-Yan Tang, G. W. F. Drake, A. T. Bondy,
A. G.
Truscott, K. G. H. Baldwin. Measurement of a helium tune-out
frequency: an independent test of quantum electrodynamics. Science,
2022; 376 (6589): 199 DOI: 10.1126/science.abk2502 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2022/04/220408083856.htm
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