New atomic clocks measure the extension of Einstein’s general relativity time on a millimeter scale

JILA physicists measured Albert Einstein’s theory of general relativity, or rather the effect called slowing down time, on the smallest scale, 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 issue of February 17, 2022 nature, suggest how to make atomic clocks 50 times more accurate than modern best designs, and suggest a way to perhaps reveal how relativity and gravity interact with quantum mechanics, a major challenge in physics.

JILA is jointly managed 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 combine quantum physics with gravity, for example, by studying complex physics when particles are distributed in different places in curved space-time,” said June E. NIST / JILA staffer. timing, it also shows that there are no obstacles to making the watch 50 times more accurate than it is today – this is fantastic news. “

JILA Redshift Atom Cloud

JILA researchers measured the slowdown of time or how the ticking frequency of atomic clocks varied with altitude in this tiny cloud of strontium atoms. Credit: Jacobson / NIST

Einstein’s general theory of relativity of 1915 explains large-scale effects such as gravitational influence on time, and has important practical applications such as the correction of GPS satellite measurements. Although the theories are over a century old, physicists are still fascinated by it. NIST scientists have used atomic clocks as sensors to more and more accurately measure relativity, which may help finally explain how its effects interact with quantum mechanics, a set of rules for the subatomic world.

According to general relativity, atomic clocks at different altitudes in a gravitational field travel at different speeds. The frequency of atomic radiation decreases – shifts toward the red end of the electromagnetic spectrum – when observed at a stronger gravity, closer to Earth. This means that at lower altitudes the clock ticks slower. This effect has been repeatedly demonstrated; for example NIST physicists measured it in 2010 comparing two independent atomic clocks, one located 33 centimeters (about 1 foot) above the other.

JILA researchers have now measured frequency shifts between the top and bottom of one sample with about 100,000 ultra-cold strontium atoms loaded into optical gratinga laboratory setup similar to a group earlier atomic clocks. In this new case, the lattice, which can be visualized as a stack of pancakes created with laser beams, has unusually large, flat, thin tortillas, and they are formed by less intense light than usual. This design reduces distortion in the lattice, usually caused by light scattering and atoms, homogenizes the sample and expands the waves of matter of atoms, the shape of which indicates the probability of atoms in certain places. The energy states of the atoms are so well controlled that they all moved in precise unison between the two energy levels for 37 seconds, a record of what is called quantum coherence.

Crucial to the new results was the Ye group image innovationwhich provides a microscopic map of the frequency distribution of the sample, and their method of comparing the two areas[{” attribute=””>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: “Resolving the gravitational redshift in a millimetre-scale atomic sample” by Tobias Bothwell, Colin J. Kennedy, Alexander Aeppli, Dhruv Kedar, John M. Robinson, Eric Oelker, Alexander Staron & Jun Ye, 16 February 2022, Nature.
DOI: 10.1038/s41586-021-04349-7

Funding was provided by the Defense Advanced Research Projects Agency, National Science Foundation, Department of Energy Quantum System Accelerator, NIST and Air Force Office for Scientific Research. New atomic clocks measure the extension of Einstein’s general relativity time on a millimeter scale

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