The design, which uses entangled atoms, could help scientists detect dark matter and study gravity's effect on time
From Massachusetts Institute of
Technology
December 16, 2020 -- A newly-designed
atomic clock uses entangled atoms to keep time even more precisely than its
state-of-the-art counterparts. The design could help scientists detect dark
matter and study gravity's effect on time.
Atomic clocks are the most precise
timekeepers in the world. These exquisite instruments use lasers to measure the
vibrations of atoms, which oscillate at a constant frequency, like many
microscopic pendulums swinging in sync. The best atomic clocks in the world
keep time with such precision that, if they had been running since the beginning
of the universe, they would only be off by about half a second today.
Still, they could be even more precise.
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 start to answer some
mind-bending questions, such as what effect gravity might have on the passage
of time and whether time itself changes as the universe ages.
Now a new kind of atomic clock designed
by MIT physicists may enable scientists explore such questions and possibly
reveal new physics.
The researchers report in the
journal Nature that they have built an atomic clock that
measures not a cloud of randomly oscillating atoms, as state-of-the-art designs
measure now, but instead atoms that have been quantumly entangled. The atoms
are correlated in a way that is impossible according to the laws of classical
physics, and that allows the scientists to measure the atoms' vibrations more
accurately.
The new setup can achieve the same
precision four times faster than clocks without entanglement.
"Entanglement-enhanced optical
atomic clocks will have the potential to reach a better precision in one second
than current state-of-the-art optical clocks," says lead author Edwin
Pedrozo-Peñafiel, a postdoc in MIT's Research Laboratory of Electronics.
If state-of-the-art atomic clocks were
adapted to measure entangled atoms the way the MIT team's setup does, their
timing would improve such that, over the entire age of the universe, the clocks
would be less than 100 milliseconds off.
The paper's other co-authors from MIT
are Simone Colombo, Chi Shu, Albert Adiyatullin, Zeyang Li, Enrique Mendez,
Boris Braverman, Akio Kawasaki, Saisuke Akamatsu, Yanhong Xiao, and Vladan
Vuletic, the Lester Wolfe Professor of Physics.
Time limit
Since humans began tracking the passage
of time, they have done so using periodic phenomena, such as the motion of the
sun across the sky. Today, vibrations in atoms are the most stable periodic
events that scientists can observe. Furthermore, one cesium atom will oscillate
at exactly the same frequency as another cesium atom.
To keep perfect time, clocks would
ideally track the oscillations of a single atom. But at that scale, an atom is
so small that it behaves according to the mysterious rules of quantum
mechanics: When measured, it behaves like a flipped coin that only when
averaged over many flips gives 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 toward 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, in
order to get an estimate of their average oscillations. A typical atomic clock
does this by first using a system of lasers to corral a gas of ultracooled
atoms into a trap formed by a laser. A second, very stable laser, with a
frequency close to that of the atoms' vibrations, is sent to probe the atomic
oscillation and thereby keep track of time.
And yet, the Standard Quantum Limit is
still at work, meaning there is still some uncertainty, even among thousands of
atoms, regarding their exact individual frequencies. This is where Vuletic and
his group have shown that quantum entanglement may help. In general, quantum
entanglement describes a nonclassical physical state, in which atoms in a group
show correlated measurement results, even though each individual atom behaves
like the random toss of a coin.
The team reasoned that if atoms are
entangled, their individual oscillations would tighten up around a common
frequency, with less deviation than if they were not entangled. The average
oscillations that an atomic clock would measure, therefore, would have a
precision beyond the Standard Quantum Limit.
Entangled clocks
In their new atomic clock, Vuletic and
his colleagues entangle around 350 atoms of ytterbium, which oscillates at the
same very high frequency as visible light, meaning any one atom vibrates
100,000 times more often in one second than cesium. If ytterbium's oscillations
can be tracked precisely, scientists can use the atoms to distinguish ever
smaller intervals of time.
The group used standard techniques to
cool the atoms and trap them in an optical cavity formed by two mirrors. They
then sent a laser through the optical cavity, where it ping-ponged between the
mirrors, interacting with the atoms thousands of times.
"It's like the light serves as a
communication link between atoms," Shu explains. "The first atom that
sees this light will modify the light slightly, and that light also modifies
the second atom, and the third atom, and through many cycles, the atoms
collectively know each other and start behaving similarly."
In this way, the researchers quantumly
entangle the atoms, and then use another laser, similar to existing atomic
clocks, to measure their average frequency. When the team ran a similar
experiment without entangling atoms, they found that the atomic clock with
entangled atoms reached a desired precision four times faster.
"You can always make the clock more
accurate by measuring longer," Vuletic says. "The question is, how
long do you need to reach a certain precision. Many phenomena need to be
measured on fast timescales."
He says if today's state-of-the-art
atomic clocks can be adapted to measure quantumly entangled atoms, they would
not only keep better time, but they could help decipher signals in the universe
such as dark matter and gravitational waves, and start to answer some age-old
questions.
"As the universe ages, does the
speed of light change? Does the charge of the electron change?" Vuletic
says. "That's what you can probe with more precise atomic clocks."
Story Source:
Materials provided
by Massachusetts
Institute of Technology. Original written by Jennifer Chu. Note:
Content may be edited for style and length.
Related Multimedia:
Journal Reference:
- Pedrozo-Peñafiel,
E., Colombo, S., Shu, C. et al. Entanglement on an optical
atomic-clock transition. Nature, 2020 DOI: 10.1038/s41586-020-3006-1
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