Every time you check the time on your phone, make an online transaction, or use a navigation app, you are depending on the precision of atomic clocks.
An atomic clock keeps time by relying on the "ticks" of atoms as they naturally oscillate at rock-steady frequencies. Today's atomic clocks operate by tracking cesium atoms, which tick over 10 billion times per second. Each of those ticks is precisely tracked using lasers that oscillate in sync, at microwave frequencies.
Scientists are developing next-generation atomic clocks that rely on even faster-ticking atoms such as ytterbium, which can be tracked with lasers at higher, optical frequencies. If they can be kept stable, optical atomic clocks could track even finer intervals of time, up to 100 trillion times per second.
Now, MIT physicists have found a way to improve the stability of optical atomic clocks, by reducing "quantum noise" - a fundamental measurement limitation due to the effects of quantum mechanics, which obscures the atoms' pure oscillations. In addition, the team discovered that an effect of a clock's laser on the atoms, previously considered irrelevant, can be used to further stabilize the laser.
The researchers developed a method to harness a laser-induced "global phase" in ytterbium atoms, and have boosted this effect with a quantum-amplification technique. The new approach doubles the precision of an optical atomic clock, enabling it to discern twice as many ticks per second compared to the same setup without the new method. What's more, they anticipate that the precision of the method should increase steadily with the number of atoms in an atomic clock.
The researchers detail the method, which they call global phase spectroscopy, in a study appearing today in the journal Nature . They envision that the clock-stabilizing technique could one day enable portable optical atomic clocks that can be transported to various locations to measure all manner of phenomena.
"With these clocks, people are trying to detect dark matter and dark energy, and test whether there really are just four fundamental forces , and even to see if these clocks can predict earthquakes," says study author Vladan Vuletić, the Lester Wolfe Professor of Physics at MIT. "We think our method can help make these clocks transportable and deployable to where they're needed."
The paper's co-authors are Leon Zaporski, Qi Liu, Gustavo Velez, Matthew Radzihovsky, Zeyang Li, Simone Colombo, and Edwin Pedrozo-Peñafiel, who are members of the MIT-Harvard Center for Ultracold Atoms and the MIT Research Laboratory of Electronics.
Ticking time
In 2020, Vuletić and his colleagues demonstrated that an atomic clock could be made more precise by quantumly entangling the clock's atoms. Quantum entanglement is a phenomenon by which particles can be made to behave in a collective, highly correlated manner. When atoms are quantumly entangled, they redistribute any noise, or uncertainty in measuring the atoms' oscillations, in a way that reveals a clearer, more measurable "tick."
In their previous work, the team induced quantum entanglement among several hundred ytterbium atoms that they first cooled and trapped in a cavity formed by two curved mirrors. They sent a laser into the cavity, which bounced thousands of times between the mirrors, interacting with the atoms and causing the ensemble to entangle. They were able to show that quantum entanglement could improve the precision of existing atomic clocks by essentially reducing the noise, or uncertainty between the laser's and atoms' tick rates.
At the time, however, they were limited by the ticking instability of the clock's laser. In 2022, the same team derived a way to further amplify the difference in laser versus atom tick rates with "time reversal" - a trick that relies on entangling and de-entangling the atoms to boost the signal acquired in between.
However, in that work the team was still using traditional microwaves, which oscillate at much lower frequencies than the optical frequency standards ytterbium atoms can provide. It was as if they had painstakingly lifted a film of dust off a painting, only to then photograph it with a low-resolution camera.
"When you have atoms that tick 100 trillion times per second, that's 10,000 times faster than the frequency of microwaves," Vuletić says. "We didn't know at the time how to apply these methods to higher-frequency optical clocks that are much harder to keep stable."
About phase
In their new study, the team has found a way to apply their previously developed approach of time reversal to optical atomic clocks. They then sent in a laser that oscillates near the optical frequency of the entangled atoms.
"The laser ultimately inherits the ticking of the atoms," says first author Zaporski. "But in order for this inheritance to hold for a long time, the laser has to be quite stable."
The researchers found they were able to improve the stability of an optical atomic clock by taking advantage of a phenomenon that scientists had assumed was inconsequential to the operation. They realized that when light is sent through entangled atoms, the interaction can cause the atoms to jump up in energy, then settle back down into their original energy state and still carry the memory about their round trip.
"One might think we've done nothing," Vuletić says. "You get this global phase of the atoms, which is usually considered irrelevant. But this global phase contains information about the laser frequency."
In other words, they realized that the laser was inducing a measurable change in the atoms, despite bringing them back to the original energy state, and that the magnitude of this change depends on the laser's frequency.
"Ultimately, we are looking for the difference of laser frequency and the atomic transition frequency," explains co-author Liu. "When that difference is small, it gets drowned by quantum noise. Our method amplifies this difference above this quantum noise."
In their experiments, the team applied this new approach and found that through entanglement they were able to double the precision of their optical atomic clock.
"We saw that we can now resolve nearly twice as small a difference in the optical frequency or, the clock ticking frequency, without running into the quantum noise limit," Zaporski says. "Although it's a hard problem in general to run atomic clocks, the technical benefits of our method it will make it easier, and we think this can enable stable, transportable atomic clocks."
This research was supported, in part, by the U.S. Office of Naval Research, the National Science Foundation, the U.S. Defense Advanced Research Projects Agency, the U.S. Department of Energy, the U.S. Office of Science, the National Quantum Information Science Research Centers, and the Quantum Systems Accelerator.
