JILA physicists have measured Albert Einstein's theory of general
relativity, or more specifically, the effect called time dilation, at the
smallest scale ever, showing that two tiny atomic clocks, separated by just
a millimeter or the width of a sharp pencil tip, tick at different rates.
The experiments, described in the Feb. 17 issue of Nature, suggest how to
make atomic clocks 50 times more precise than today's best designs and offer
a route to perhaps revealing how relativity and gravity interact with
quantum mechanics, a major quandary in physics.
JILA is jointly operated by the National Institute of Standards and
Technology (NIST) and the University of Colorado Boulder.
"The most important and exciting result is that we can potentially connect
quantum physics with gravity, for example, probing complex physics when
particles are distributed at different locations in the curved space-time,"
NIST/JILA Fellow Jun Ye said. "For timekeeping, it also shows that there is
no roadblock to making clocks 50 times more precise than today—which is
fantastic news."
Einstein's 1915 theory of general relativity explains large-scale effects
such as the gravitational effect on time and has important practical
applications such as correcting GPS satellite measurements. Although the
theory is more than a century old, physicists remain fascinated by it. NIST
scientists have used atomic clocks as sensors to measure relativity more and
more precisely, which may help finally explain how its effects interact with
quantum mechanics, the rulebook for the subatomic world.
According to general relativity, atomic clocks at different elevations in a
gravitational field tick at different rates. The frequency of the atoms'
radiation is reduced—shifted toward the red end of the electromagnetic
spectrum—when observed in stronger gravity, closer to Earth. That is, a
clock ticks more slowly at lower elevations. This effect has been
demonstrated repeatedly; for example, NIST physicists measured it in 2010 by
comparing two independent atomic clocks, one positioned 33 centimeters
(about 1 foot) above the other.
The JILA researchers have now measured frequency shifts between the top and
bottom of a single sample of about 100,000 ultracold strontium atoms loaded
into an optical lattice, a lab setup similar to the group's earlier atomic
clocks. In this new case the lattice, which can be visualized as a stack of
pancakes created by laser beams, has unusually large, flat, thin cakes, and
they are formed by less intense light than normally used. This design
reduces the distortions in the lattice ordinarily caused by the scattering
of light and atoms, homogenizes the sample, and extends the atoms' matter
waves, whose shapes indicate the probability of finding the atoms in certain
locations. The atoms' energy states are so well controlled that they all
ticked between two energy levels in exact unison for 37 seconds, a record
for what is called quantum coherence.
Crucial to the new results were the Ye group's imaging innovation, which
provided a microscopic map of frequency distributions across the sample, and
their method of comparing two regions of an atom cloud rather than the
traditional approach of using two separate clocks.
The measured redshift across the atom cloud was tiny, in the realm of
0.0000000000000000001, consistent with predictions. (While much too small
for humans to perceive directly, the differences add up to major effects on
the universe as well as technology such as GPS.) The research team resolved
this difference quickly for this type of experiment, in about 30 minutes of
averaging data. After 90 hours of data, their measurement precision was 50
times better than in any previous clock comparison.
"This a completely new ballgame, a new regime where quantum mechanics in
curved space-time can be explored," Ye said. "If we could measure the
redshift 10 times even better than this, we will be able to see the atoms'
whole matter waves across the curvature of space-time. Being able to measure
the time difference on such a minute scale could enable us to discover, for
example, that gravity disrupts quantum coherence, which could be at the
bottom of why our macroscale world is classical."
Better clocks have many possible applications beyond timekeeping and
navigation. Ye suggests atomic clocks can serve as both microscopes to see
minuscule links between quantum mechanics and gravity and as telescopes to
observe the deepest corners of the universe. He is using clocks to look for
mysterious dark matter, believed to constitute most matter in the universe.
Atomic clocks are also poised to improve models and understanding of the
shape of the Earth through the application of a measurement science called
relativistic geodesy.
Reference:
Tobias Bothwell, Resolving the gravitational redshift in a millimetre-scale
atomic sample, Nature (2022).
DOI: 10.1038/s41586-021-04349-7.
Related: Shimon Kolkowitz, Differential clock comparisons with a multiplexed
optical lattice clock, Nature (2022).
DOI: 10.1038/s41586-021-04344-y.
Tags:
Physics