Breaking The Laser Stability Record Using New Crystalline Mirrors

In a mirror maze, finding yourself between two mirrors is designed to leave you disoriented and feeling a little unstable. In contrast, getting caught between two mirrors can be incredibly stabilizing for laser light. Scientists make lasers with incredibly stable frequencies by using optical cavities, which are mirrored chambers where light bounces back and forth hundreds of thousands of times.

Researchers at JILA have a long history of improving laser technologies and working with optical cavities. While pushing the limits of laser stability and precision, they have found a plethora of potential disturbances that they have to address to maintain stable frequencies. A tiny vibration, such as from a shaking pump in the lab, can negatively impact the operation of an optical cavity if unchecked.

A team of researchers, led by JILA and National Institute of Standards and Technology Fellow and University of Colorado Boulder Physics professor Jun Ye, has been pushing the limits of stable laser technology for more than two decades, and the team has seen signs that the natural motion of atoms that make up the mirror coatings limit their performance. Overcoming this effect and improving the stability of lasers could unlock new opportunities for experiments, like gravitational wave detectors, and improved technologies, like better atomic clocks.

So, the researchers sought an improved mirror coating. In recent experiments, Ye and his group have collaborated with a team led by Thomas Legero and Uwe Sterr at the Physikalisch-Technische Bundesanstalt in Germany; together, the researchers have tested a new style of crystalline mirror coating expected to mitigate the negative impact of the ways atoms collectively move in the mirror’s structure. In an article published in the journal Physical Review Letters on Jan. 20, 2026, they described the experiment and the unparalleled stability the new coatings allowed them to achieve.

“So far, it had never been demonstrated that these coatings can support superior performance at the state-of-the-art level,” says Dahyeon Lee, a JILA postdoctoral researcher and first author of the article. “This work actually shows that these crystalline coatings give you four times better performance than traditional mirror coatings, while at the same time demonstrating the lowest instability of all optical cavities.” 

Optical cavities are so useful in making precision lasers because light wants to naturally fall into certain frequencies when it is trapped between two reflective walls. A particular distance between two mirrors will support certain frequencies while discouraging others. But any vibration of the mirrors or any stretching or contracting of the chamber can interfere with the process and prevent the light from settling precisely into a specific frequency.

Members of Ye’s lab have long ago addressed the most obvious disruptions—like the vibrations of the cooling system that is necessary to keep the cavity working optimally. By using excellent equipment and being vigilant about tamping down vibrations, they have reached a point where things normally run so smoothly that they can see signs of their performance being impaired by the collective motion of all the atoms making up the mirror coating used in the cavity. Inside any solid object, atoms aren’t perfectly still, but depending on the structure of the material, they can all coordinate their motion in particular ways. Certain disturbances of a laser can be dealt with just by averaging the laser’s frequency for a certain amount of time, but the collective movement of the atoms in the mirrors couldn’t be dealt with so easily.

“This is a very special experiment where you can think about both engineering and physics,” says Zoey Hu, a JILA graduate student and author of the article. “What we're really doing here sounds like a simple thing—you're just keeping two mirrors as stable as possible with respect to each other. But when it comes to doing just that one simple thing, there are actually so many little details you have to think about and address.”

To address the collective atomic motion, one of the details the team has considered is how atoms behave in different materials. The new crystalline mirror coatings are made of aluminum, gallium and arsenic and have a structure that keeps the atoms locked more tightly in place than the atoms in the established coatings, which are made from silicon dioxide and tantalum pentoxide and have a more amorphous structure. The strict crystalline structure of the new coatings means the atom’s collective motion experiences less natural loss of energy and fewer random fluctuations in their motion, which should improve the stability of the frequency in experiments.

To show that the coatings were competitive with existing state-of-the-art technologies, the group had to put in some work, including installing the mirror coatings in a high-quality silicon cavity, cooling the cavity down to its frigid optimal temperature (17 K) and ensuring that the system operated smoothly. All their efforts paid off, and the system delivered a more stable frequency than the established coatings could. The coatings require some additional effort to work with, but the results show that the effort can deliver increased stability when the need arises.

“With this technology, and because we already have some other nice cavities, we can show better performance than you could get from any other laser in the world,” says Ben Lewis, a JILA postdoctoral researcher and author of the article. “The crystalline coatings are harder to work with. They're more finicky. But if you want to push and get better performance, they're one of the ways that you can.”

Lewis went on to say that the frequency is tied to the average distance the light travels between reflections and that the stability of their laser frequency averaged over a period of 10 seconds translates into knowing the length of the light’s journey to less than 1 percent of the width of a proton.

Since the coatings produced such great results, the group combined them with another technique that is known to be useful in increasing the stability of a laser frequency when another laser at the same frequency is available. They performed a process, called optical frequency averaging, where two cavities are simultaneously used and the frequency is averaged together. The other cavity used conventional coatings, but its length is more than three times longer, which is an alternative approach to increasing a cavity’s frequency stability. They demonstrated that the technique could increase the resulting frequency stability even further.

The group also shared data they collected that showed how the frequencies of four cryogenic silicon cavities have slowly changed over time. These cavities, located at either JILA or PTB, achieve the best performance currently possible for stable lasers. The frequency observed for each cavity naturally drifts after it is assembled, but over time, the drifting slows down. The data showed the changes of two cavities with the new mirror coatings and two with the established coatings. The exact role the coatings play in producing the drift remains a mystery, but the new data provides clues and indicates that the cavities with new coatings stabilized more quickly than the more established coatings.

While the group has already set a new record for laser frequency stability with the setup, the team is optimistic that the approaches used in these experiments will deliver even better results in the future. They are continuing to observe the cavity with the new coatings to see how it behaves in the long run and to use the cavity in new experiments, including applying it to keeping time.

“We know these cavities are stable and may be much better than the traditional way of doing timekeeping,” Lee says. “We're trying to reimagine how timekeeping can be done in the future by using these silicon cavities as a stable ticking machine.” 
 

Written by Bailey Bedford, Freelance Science Communicator