Atoms are trapped in an optical cavity composed of two mirrors. When a “squeezing” laser is set through the cavity, the atoms are tangled and their frequency is measured with a second laser as a platform for more accurate atomic clocks. Credit: courtesy of researchers
The new atomic clock design, which uses tangled atoms, could help scientists detect dark matter and study the effect of gravity on time.
Atomic clocks are the most accurate chronometers in the world. These refined instruments use lasers to measure the vibrations of atoms, which oscillate at a constant frequency, like many microscopic pendulums that connect synchronously. The best atomic clocks in the world keep time with such precision that, if they had worked since the beginning of the universe, they would have been stopped for only about half a second today.
However, it could be even more accurate. If atomic clocks could more accurately measure atomic vibrations, they would be sensitive enough to detect phenomena such as dark matter and gravitational waves. With better atomic clocks, scientists could also begin to answer a few questions that question us, such as what effect gravity might have on the passage of time and whether time itself changes as the universe ages.
Now a new type of atomic clock designed by WITH physicists can allow scientists to explore such questions and possibly reveal new physics.
Researchers report in the journal today The nature that they built an atomic clock that does not measure a cloud of randomly oscillating atoms, as state-of-the-art projects now measure, but instead atoms that have been quantum entangled. Atoms are correlated in an impossible way according to the laws of classical physics and that allows scientists to measure the vibrations of atoms more accurately.
The new configuration can achieve the same accuracy four times faster than clutter-free watches.
“Tangle-enhanced optical atomic clocks will have the potential to achieve better accuracy in a second than state-of-the-art optical clocks,” says lead author Edwin Pedrozo-Peñafiel, a postdoc in MIT’s electronic research lab.
If state-of-the-art atomic clocks were adapted to measure tangled atoms as configured by the MIT team, their timing would be improved so that, across the entire age of the universe, the clocks would be less than 100 milliseconds apart.
The other co-authors of MIT are Simone Colombo, Chi Shu, Albert Adiyatullin, Zeyang Li, Enrique Mendez, Boris Braverman, Akio Kawasaki, Saisuke Akamatsu, Yanhong Xiao and Vladan Vuletic, physics professor Lester Wolfe.
Limit
Ever since humans began to watch the passage of time, they have done so using periodic phenomena, such as the movement of the sun in the sky. Today, vibrations in atoms are the most stable periodic events that scientists can observe. Moreover, a cesium atom it will oscillate at exactly the same frequency as another cesium atom.
To keep time perfect, watches would ideally track the oscillations of a single atom. But at this scale, an atom is so small that it behaves according to the mysterious rules of quantum mechanics: when measured, it behaves like an inverted coin that only when mediated on several flips, provides the correct probabilities. This limitation is what physicists refer to as the standard quantum limit.
“When you increase the number of atoms, the average given by all these atoms goes to something that gives the correct value,” says Colombo.
This is why today’s atomic clocks are designed to measure a gas composed of thousands of the same type of atom, to get an estimate of their average oscillations. A typical atomic clock does this by first using a laser system to coral a gas of ultracool atoms into a trap formed by a laser. A second laser, very stable, with a frequency close to that of the vibrations of the atoms, is sent to test the atomic oscillation and thus to keep track of time.
And yet, the standard quantum limit is still at work, which means that there is still some uncertainty, even among thousands of atoms, about their exact individual frequencies. Here Vuletic and his group have shown that quantum entanglement can help. In general, quantum entanglement describes a non-classical physical state, in which the atoms in a group show correlated measurement results, even if each individual atom behaves like a random toss of a coin.
The team argued that if the atoms were tangled, their individual oscillations would tighten around a common frequency, with a smaller deviation than if they were not tangled. Therefore, the average oscillations that an atomic clock would measure would have an accuracy beyond the standard quantum limit.
Tangled watches
In their new atomic clock, Vuletic and colleagues confuse about 350 ytterbium atoms, which oscillate at the same very high frequency as visible light, meaning that any atom vibrates 100,000 times more often in a second than cesium. . If the oscillations of the yterter can be accurately tracked, scientists can use atoms to distinguish smaller and smaller time intervals.
The group used standard techniques to cool atoms and trap them in an optical cavity consisting of two mirrors. Then they sent a laser through the optical cavity, where he ping-ponged between the mirrors, interacting with atoms a thousand times.
“It’s like light serves as a communication link between atoms,” Shu explains. “The first atom that sees this light will slightly change the light, and that light changes the second atom and the third atom, and through many cycles, the atoms know each other collectively and begin to behave similarly.”
In this way, researchers quantically confuse atoms and then use another laser, similar to existing atomic clocks, to measure their average frequency. When the team performed a similar experiment without confusing atoms, they found that the atomic clock with tangled atoms reached the desired accuracy four times faster.
“You can always make the clock more accurate by measuring more,” says Vuletic. “The question is, how long do you need to reach a certain accuracy. Many phenomena must be measured on fast time scales. “
He says that if state-of-the-art atomic clocks can be adapted to measure quantum entangled atoms, it would not only keep time better, but could help decipher signals in the universe, such as dark matter and gravitational waves, and begin to answer some old questions.
“As the universe ages, does the speed of light change?” Is the electron charge changing? Says Vuletic. “That’s what you can probe with more accurate atomic clocks.”
Reference: “Entanglement on an optical atomic-clock transition” by Edwin Pedrozo-Peñafiel, Simone Colombo, Chi Shu, Albert F. Adiyatullin, Zeyang Li, Enrique Mendez, Boris Braverman, Akio Kawasaki, Daisuke Akamatsu, Yanhong Xiao and Vladan Vuletić, December 16, 2020, The nature.
DOI: 10.1038 / s41586-020-3006-1
This research was supported, in part, by DARPA, National Science Foundation and Office of Naval Research.