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 color 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 color 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 Ph.D. 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 Ph.D. 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 color was varied.
He realized this effect could be harnessed to very accurately determining
the precise color 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 color 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.
Reference:
B. M. Henson et al, Measurement of a helium tune-out frequency: an
independent test of quantum electrodynamics, Science (2022).
DOI: 10.1126/science.abk2502
Tags:
Physics