Research Highlights

An Atomic Clock That Stays Cool and Can Rock and Roll Without Losing Time

Steven Burrows — Thu, 09 Apr 2026 15:07:45 +0000

An Atomic Clock That Stays Cool and Can Rock and Roll Without Losing Time Steven Burrows Thu, 04/09/2026 - 09:07

Bailey Bedford / Freelance Science Communicator

A new proposal shows how guiding atoms through a controlled loop of low-energy states using an additional atomic state and a second color of light can eliminate the heating that has long hindered superradiant atomic clocks. The design also makes the laser more robust to vibrations, as coordinated interactions among atoms help keep them synchronized even when the cavity is disturbed.

In popular culture, lasers are often portrayed as portable blasters that superheat whatever they hit. Some lasers do deliver tremendous amounts of energy in reality, but for scientists and engineers, lasers often need to do more than deliver just raw power. They need to deliver a very precise frequency—color—of light.

Precise lasers open many opportunities for experiments and technologies, notably atomic clocks, which offer the most precise timekeeping in the world. Atomic clocks are used in experiments, such as searches for dark matter, and they also make possible everyday technologies, like GPS. Currently, the most precise lasers, and therefore the most precise atomic clocks, are bulky and can be disrupted by small vibrations or changes in temperature, which limits their applications.

In an article published April 9, 2026, in the journal Physical Review Letters, JILA graduate student Jarrod Reilly proposed a new laser design that may allow for greater precision while making lasers more compact and robust. The design was developed along with JILA Fellows Murray Holland and John Cooper, as well as Simon Jäger—who was formerly a JILA postdoctoral researcher and is now an international collaborator at the University of Bonn in Germany. It builds on prior research they and their colleagues at JILA have performed, and their analysis indicated that it solves multiple problems that have limited past experiments. The improvements suggest a way that future atomic clocks can be both more precise and more convenient.

“Time and frequency are the two physical quantities that humans can measure the best,” Holland says. “This high sensitivity allows us to make measurements that are incredibly precise. Pushing it further opens up new domains where we could look farther than we've ever been able to look before.”

The new design is for a type of laser called a superradiant laser, and having a reliable superradiant laser is necessary to create a new type of compact atomic clock called an active atomic clock. Superradiant lasers that could enable active atomic clocks were first proposed by JILA researchers in 2009, and JILA researchers continue to refine the technology. Active atomic clocks use similar principles to standard atomic clocks but include some important tweaks.

Both traditional and active atomic clocks take advantage of the fact that atoms have quantum states which researchers can link together using light. Light comes in quantum packets that each carry a certain amount of energy that corresponds to its frequency—how quickly the light waves oscillate. An atom can be pushed from its initial state into a higher-energy state by hitting it with light of the right frequency. An atom with extra energy will sometimes release light to return to a lower-energy state. The consistent waves of light associated with a particular transition between chosen high- and low-energy atomic states can play a role similar to the steady swinging of a pendulum in a grandfather clock.

Traditional atomic clocks shine a laser on atoms and monitor when the atoms interact with the light at the correct frequency. An active atomic clock, instead, uses many atoms releasing light to create a laser with the desired frequency.

Making an active atomic clock requires getting all the atoms to work together to produce the superradiant laser. If too few atoms emit light at a time, nothing will be observed, and if different atoms simultaneously emit light in the wrong way, the resulting wave that is generated may lose coherence and become unusable.

To coordinate atoms, researchers put them in a special cavity where light bounces between two mirrors. The cavity maintains the frequency of light needed to interact with the atoms and encourages them to synchronize. The process resembles performers coordinating their dance steps by all listening to the same music.

In 2012, Holland collaborated with JILA Fellow James Thompson and demonstrated in experiments that superradiant lasers worked. But there was a hiccup: The process only worked for short periods at a time, and the laser ended up as a series of pulses, which couldn’t be used directly as an active atomic clock. The chamber coordinated the atoms releasing the desired frequency of light. However, when the atoms were put into the chosen energetic state, each atom emitted a small amount of extra light without any coordination. This unpredictable emission resulted in random motion that heated the atoms and eventually disrupted the synchronization needed for superradiance.

The new proposal suggests a method to eliminate the heating. Reilly, who is the first author of the paper, realized the atoms could be guided throughout the entire process and avoid the heating. Reilly observed that utilizing an additional state in the atom allows an experiment to use a different color of light to direct atoms through the troublesome step.

To make it work, he had to select an atom with two very similar states when the atom has as little energy as possible. Researchers can supply light to move the atoms between the two low-energy states. Then, placing the atoms in that additional low-energy state allows a second color of light to be introduced into a cavity that coordinates how the atoms move to the selected energetic state.

Now, the atoms are guided through more than the single dance step of producing the desired frequency. The experiment directs the atoms through a full loop of states, with a scientist controlling where all the energy goes. Each step is carefully managed, and the extra energy is predictably directed away from the atoms, where it can be easily handled.

The group used ideas from particle physics to develop a simulation of the quantum process that Reilly had identified. The simulation showed that the process should eliminate the heating that had previously prevented the creation of active atomic clocks using superradiant lasers.

“This heating rate should be so low that it would be easily manageable in a real apparatus,” Holland says.

But they went beyond eliminating the heating problem. They also discovered that the new design made the laser less sensitive to the shaking of the chamber than prior methods. The atoms didn’t just interact with the light in the cavity but with each other, like performers who can hear each other singing to the music. The new controlled transitions and extra light bouncing back and forth in the cavity should help the atoms interact and remain coordinated. If the cavity is slightly disrupted, it is like the music temporarily cutting out or being distorted, but the singing helps keep the performers coordinated nonetheless.

With increased coordination, the atoms should depend largely on synchronization with each other and less on the cavity, so shaking the cavity shouldn’t have much effect. The researchers used the simulation to show that there are certain ways to set up the experiment in which the frequency of the laser is not sensitive to vibrations of the cavity’s mirrors at all.

“What they're measuring in a clock is that frequency,” Reilly says. “The big-game-changer is that it becomes completely insensitive to vibrations, which people have spent 20 years trying to overcome. You could jump up and down next to the experiment, and in a regular clock, you'd see the color change, but you can jump up and down next to our clock and not see the color change. It should stay stable.”

The researchers also used their simulations to show that even when individual atoms fall out of sync with the others, it shouldn’t disrupt the superradiance—a known problem with some previous methods.

The team says they hope to see the proposal realized in an experiment, and they also want to combine their idea with another concept for the next generation of clocks: nuclear clocks. Nuclear clocks are similar to atomic clocks but use the quantum states of nuclei. The researchers believe their new superradiance technique could solve a lingering issue with nuclear clocks and provide a path to a new generation of unprecedentedly accurate timepieces.

Researchers at JILA propose a new superradiant laser design for next-generation “active” atomic clocks that eliminates atom-heating and vibration sensitivity, two major obstacles that have limited precision and practicality. By carefully guiding atoms through a controlled loop of quantum states, the approach could enable compact, robust atomic—and potentially nuclear—clocks that maintain extreme accuracy even under physical disturbances.

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Breaking The Laser Stability Record Using New Crystalline Mirrors

Steven Burrows — Wed, 18 Feb 2026 15:25:03 +0000

Breaking The Laser Stability Record Using New Crystalline Mirrors Steven Burrows Wed, 02/18/2026 - 08:25

Bailey Bedford / Freelance Science Communicator

A Crystalline Coated 6cm Silicon Cavity. Image credit: Steven Burrows / JILA

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.”

JILA researchers, working with collaborators in Germany, demonstrated that new crystalline mirror coatings dramatically reduce atomic-level noise in optical cavities, enabling lasers with record‑breaking frequency stability. By outperforming traditional coatings by a factor of four, these mirrors open the door to more precise experiments and future advances in technologies such as atomic clocks and gravitational‑wave detection.

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Nuclear Clockwork: Experiments Highlight Reproducibility of Nuclear Transition Frequency

Steven Burrows — Fri, 06 Feb 2026 18:32:10 +0000

Nuclear Clockwork: Experiments Highlight Reproducibility of Nuclear Transition Frequency Steven Burrows Fri, 02/06/2026 - 11:32

Artistic representation of a ²²⁹Th nucleus hosted inside a CaF2 crystal experiencing a local electric field gradient. The ²²⁹Th nuclear electric quadrupole moment interacts with the electric field, leading to split energy levels. Image credit: Steven Burrows / JILA

To be useful, clocks need to be consistent. Imagine two spies who synchronize their watches; they rely on them agreeing days or months later, even if one of them must take a frigid hike through arctic tundra. In many experiments, scientists similarly require that their clock is accurate to a tiny sliver of a second and that it will work the same as their colleague’s clock on the other side of the world.

Currently, when keeping time really counts, scientists and engineers turn to atomic clocks. Atomic clocks use the physics that governs the interactions between electrons and light. They can be so accurate that they could run for tens of billions of years without getting off by a second. These clocks have been used for research, such as experiments studying quantum many-body physics and relativity, and have enabled technologies, including GPS. But scientists are not satisfied. Researchers are exploring the potential of nuclear clocks to use the same principles to deliver even more precise results or to fit into an even smaller device.

JILA has been a leader in atomic clock and nuclear clock research, and in 2024 a team of researchers, led by JILA and National Institute of Standards and Technology Fellow and University of Colorado Boulder Physics professor Jun Ye, reported crucial research where they measured the first high-resolution spectrum of the nuclear transition of thorium and determined the absolute frequency of the transition. Ye and other scientists hope these transitions of thorium nuclei will be the ticking hearts of future nuclear clocks. However, there is still a lot for scientists to learn before nuclear clocks have a chance at becoming the gold standard for precision time keeping. For instance, researchers need to understand how nuclear transitions respond to things like changes in temperature, make sure that nuclear clocks can be made with a shared reproducible frequency and determine if they remain reliable over extended periods of time.

In new experiments, Ye and his colleagues have looked at crystals containing thorium to better understand how they might be used in nuclear clocks, including testing three crystal samples many times over the course of a year to check if their properties unexpectedly fluctuated over that time. In an article published in the journal Nature on January 28, 2026, they described the stability of three crystals observed over the course of multiple months, how the crystals responded to temperature changes, and how the different concentrations of thorium in each crystal affected their properties. The results revealed that the crystals have a promising stability and reproducibility and provided insights into future experiments and how similar crystals might be incorporated into high quality clocks.

“Checking frequency reproducibility, both between different host crystals and over an extended period of time, is the first step towards a systematic evaluation of the performance of the nuclear clock,” says Ye.

The group studied three crystals fabricated by Thorsten Schumm’s lab at the Technical University of Vienna. Each crystal was made of calcium fluoride but with some of the calcium atoms replaced with thorium atoms. The crystals each contained different concentrations of thorium. When the thorium atoms are in their lowest energy quantum state, Ye’s group can observe how they interact with particular frequencies of light to make their nucleus jump to higher energy states. They found that there are five transitions that they can trigger with slightly different frequencies of light. The frequencies of these transitions are critical to using thorium in a nuclear clock.

“It’s critical that Thorsten’s lab has provided three different Thorium-doped crystals, which allowed us to study the line width broadening mechanisms and the level of line center reproducibility,” says Ooi.

These interactions and frequencies follow essentially the same physics as the transitions of atoms used in atomic clocks. However, the states of the nucleus are less sensitive to fluctuations of the electric and magnetic fields around them than the states of atoms. Additionally, the nuclear states can be used even when the atoms are embedded in a crystal, unlike the states used for atomic clocks; this difference allows a nuclear clock using a crystal to have a clearer signal by using many more of the relevant atoms while perhaps also being packaged in a smaller device.

Ye’s lab previously studied how one of these crystals behaved at three different temperatures. In the new article, they continued to look at that crystal along with two others with lower concentrations of thorium.

The researchers observed that over the course of the year the properties of the first crystal were stable. The two additional crystals demonstrated the same frequency as the first and also delivered reproducible results when repeated measurements were made months apart. The fluctuations the team observed were stable to around a tenth of a trillionth of the frequency of the measured transition and are limited by the experiment’s measurement precision. These results are promising for researchers to be able to use such crystals to fabricate reliable clocks.

“We are able to show that even over the span of almost a year, we can measure the nuclear transition frequency in these crystals over and over again, and they're very consistent,” says Tian Ooi, a graduate student at JILA and first author of the paper.
The team did find some variations in the crystals’ performances based on the concentration of thorium. While the thorium all interacted with light of the same wavelength, how precisely they responded to the specific frequency varied. The state’s transition will sometimes respond to nearby frequencies and the group defines this extended range of interaction frequencies as the “line width” of the transition.

The group found that the line widths were considerably wider than theoretical calculations had predicted and that they depended on the thorium concentration with greater amounts of thorium producing broader line widths. The researchers propose that the broadening of the width may be caused by the substitution of thorium creating a subtle microstrain in the crystal’s structure that influences the nuclear transitions by making the electric field vary unevenly inside the material.

“This was an unexpected surprise,” says Ooi. “People didn’t anticipate how large this microstrain effect would be.”

Further research is needed to explain the effect and determine if it can be eliminated. Minimizing the line width is a critical factor in designing a high-performance nuclear clock, but high concentrations will also help researchers get a clear signal. So, researchers need to understand this relationship and, if possible, produce crystals with narrower line widths.

The group also continued their research into how the nuclear transition of thorium varied with temperature. They took measurements at more temperatures than they previously had, and for all three crystals, they looked at both the transition that varied the most and the transition that varied least with changes in temperature. The researchers found that the frequencies of the crystals were consistent with each other and identified the point where the material’s changes in response to temperature shift from decreasing the frequency to increasing it, which is where the impact of any temperature fluctuation is smallest. This temperature will likely be the most practical temperature to keep the crystal at when operating a nuclear clock.

The experiments also let the team map out the response of the transition that varies the most with temperature. Based on the results, the researchers suggest that in the future nuclear clocks can monitor that more sensitive frequency to record the temperature so that fluctuations to the least sensitive transition can be rapidly corrected.

Now that the group has these insights, they plan to continue studying these crystals, investigate why the line widths vary between crystals and chart a path to a future with nuclear clocks as a valuable timekeeping tool.

“I think what this paper shows is that we're moving from measuring the clock transition to really investigating how good this clock can be,” Ooi says. “There’s still interesting things to figure out, but this is one of the big steps that we have to take to show that solid-state nuclear clocks are viable.”

_{The authors acknowledge funding support from National Science Foundation QLCI OMA-2016244, DOE quantum center of Quantum System Accelerator, Army Research Office (W911NF2010182), Air Force Office of Scientific Research (FA9550-19-1-0148), National Science Foundation PHY-2317149, and National Institute of Standards and Technology. Part of this work has been funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant Agreement No. 856415) and the Austrian Science Fund (FWF) [Grant DOI: 10.55776/F1004, 10.55776/J4834, 10.55776/ PIN9526523]. The project 23FUN03 HIOC [Grant DOI: 10.13039/100019599] has received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Program and by the Participating States. We thank the National Isotope Development Center of DoE and Oak Ridge National Laboratory for providing the Th-229 used in this work.}

JILA researchers have taken a major step toward realizing next‑generation nuclear clocks by studying how thorium‑doped crystals behave over time. In new experiments published in Nature, the team tracked the stability, temperature response, and reproducibility of three calcium‑fluoride crystals containing different concentrations of thorium. Over nearly a year of measurements, all three crystals demonstrated remarkably stable nuclear transition frequencies—an essential requirement for building reliable nuclear clocks.

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JILA Researchers Overturn 25-Year-Old Explanation of Benzene Formation in Space

Steven Burrows — Fri, 09 Jan 2026 18:21:00 +0000

JILA Researchers Overturn 25-Year-Old Explanation of Benzene Formation in Space Steven Burrows Fri, 01/09/2026 - 11:21

Bailey Bedford / Freelance Science Communicator

Interstellar formation of PAHs terminates at C6H5+. Image credit: Steven Burrows / JILA

Space is famously empty. The cold vacuum of space—or more specifically, the interstellar medium—lacks much of anything, including the air needed to conduct sound. But it isn’t quite completely empty. While it’s vacant compared to what we experience in daily life, there are occasional atoms and molecules spread throughout it.

Those atoms and molecules mean that there is chemistry in space, although it doesn’t always resemble the dense, warm reactions that routinely occur in a chemist’s test tubes. One aspect of chemistry in space that researchers are interested in is the formation of polycyclic aromatic hydrocarbons (PAHs), which are molecules of carbon and hydrogen that make a broad array of chemicals on earth and in the void of space. Researchers have seen signs of light interacting with a variety of these molecules in space and being absorbed—leaving a distinctive fingerprint in the remaining light that reaches Earth. These molecules are estimated to contain somewhere between a tenth and a quarter of the carbon spread across the interstellar medium, and the molecules’ foundational building blocks are benzene (C₆H₆)—a ring of six carbon atoms, each holding a hydrogen atom.

Since 1999, researchers have had a model that they thought explained how benzene formed from smaller molecules. However, the challenges of performing experiments at the low temperatures and densities involved in mimicking the conditions in the interstellar medium have meant that researchers have relied on their theoretical understanding of the process and haven’t thoroughly tested it in experiments.

Now, JILA Fellow and University of Colorado Boulder Physics Professor Heather Lewandowski and members of her lab have used tools developed in physics laboratories to recreate the necessary conditions and have investigated how the chemistry plays out. The team described their experiment in an article published in the journal Nature Astronomy in May 2025. When they tested the process, the first steps played out as expected, but then they were surprised to find that the benzene failed to form at the final step. Their results give scientists a new window into how chemistry occurs in the interstellar medium and reopens the question of how carbon gets caught up in PAHs throughout space.

The key to recreating the chemistry occurring in the interstellar medium was creating a vacuum in a chamber and using lasers to cool molecules and hold them in place in the vacated space. This required the researchers to look at just a small number of molecules and to set aside the beakers and test tubes that are stereotypical of chemistry and instead rely on large metal chambers, air pumps, laser beams and many mirrors and lenses.

“It's a laboratory full of lasers, and vacuum chambers, and optics,” Lewandowski says. “It fills up half a room to be able to cool down these hundred little molecules.”

Selecting the right color of laser and aligning the beams correctly allows the researchers to suspend—trap—particles in a vacuum chamber as well as cool them down through a process called laser cooling. Laser cooling relies on the fact that light can give atoms and molecules a shove to slow them down and that the interaction can be tailored to depend on how the particles are moving. Carefully applied, laser cooling can get molecules down to temperatures just above absolute zero.

“Laser cooling and trapping has really been in the domain of physicists,” Lewandowski says. “The nice thing about JILA is we have physicists and chemists working together. In my own group, we have both backgrounds, and so we have the tools now that can answer these questions that really chemists didn't have the technology to tackle and physicists didn't know it was an interesting question to answer.”

These techniques allow them to focus on a small number of molecules and get a close look at the interactions that normally are obscured in a chaos of many reactions occurring rapidly and simultaneously.

With the equipment creating the needed conditions, the group started following the proposed recipe for creating benzene in the interstellar medium. The recipe’s main ingredient is a molecule of two carbon atoms and two hydrogen atoms, called acetylene (C₂H₂). The first step is mixing acetylene with molecules containing two nitrogen atoms and one hydrogen atom (N₂H⁺). The nitrogen atoms can provide their hydrogen atom to create new molecules with two carbon and three hydrogen atoms. That opens the door to two more steps of interactions with acetylene molecules to produce a molecule with six carbon atoms and five hydrogen atoms (C₆H₅⁺)—just one hydrogen short of the target benzene ring. The exact behavior of this molecule is not thoroughly understood, but the established recipe proposed that it could form benzene by capturing a molecule made from a pair of hydrogens and then letting the excess atoms go.

The team supplied just enough of the needed ingredients in the chamber so that it was improbable that more than two molecules would be reacting at a time. Using laser cooling, they cooled the molecules in the chamber down to just a few degrees Kelvin. This setup let them recreate what happens when two lonely molecules finally come together in space and get the chance to interact.

The group repeatedly ran the experiment, stopping after different amounts of time to eject the cloud of molecules and check which molecules had been formed. They saw the mixture progress through the expected steps of the recipe. They observed increases of various molecules as they were created and then decreases as they were consumed in the construction of even larger molecules. But as they waited progressively longer and longer, they never caught sight of any benzene rings. The mixture in the chamber eventually just reached a steady amount of C₆H₅⁺, and the final step of the recipe failed to occur.

“Initially we were very confused—and a little irritated—because we could never get the final reaction to happen,” says JILA postdoctoral researcher G. Stephen Kocheril, the lead author of the paper.

After performing several runs of the experiment and analyzing the data, the team concluded that the expected chain of events wasn’t happening and there must be something else occurring to produce all the benzene in space.

“None of the models now actually predict what's out there,” Lewandowski says. “If you look at observations of how many of these molecules we have out there, no model works. So we sort of said, ‘this model isn't it.’ We don't have a new model yet; that's what we're working on now. So it was kind of big for the community because it changed how larger and larger carbon-containing molecules are formed in space.”

Moving beyond the old explanation gives chemists insights into how they should think about the formation of these molecules and provides astronomers with new clues about which molecules they should be keeping an eye out for if they want to understand the chemistry happening out in the interstellar medium.

JILA Fellow and University of Colorado Boulder Physics Professor Heather Lewandowski and members of her lab have shattered a 25-year-old theory about how benzene forms in the interstellar medium, revealing that the long-accepted chemical recipe doesn’t work under space-like conditions. Their groundbreaking laser-cooling experiments open a new chapter in understanding the origins of complex carbon molecules in the cosmos.

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Narrowing In: Cooling Molecules with Light Like Never Before

Steven Burrows — Tue, 23 Dec 2025 18:23:49 +0000

Narrowing In: Cooling Molecules with Light Like Never Before Steven Burrows Tue, 12/23/2025 - 11:23

Kenna Hughes-Castleberry / JILA Science Communicator

Narrowline Laser Cooling and Spectroscopy of Molecules via Stark States. Image credit: Steven Burrows / JILA

Atoms have long been the cornerstone of laser cooling experiments. Their relatively simple structure makes them straightforward to cool with light, allowing scientists to achieve temperatures near absolute zero. Molecules, by contrast, present a much more formidable challenge. With complex rotational, vibrational, and electronic states, they’re significantly harder to tame.

Now, in a study published in Physical Review X Quantum, a team led by JILA and NIST Fellow and University of Colorado Boulder physics professor Jun Ye has demonstrated—for the first time—narrowline laser cooling of a molecule. By utilizing a previously unaddressed transition in the diatomic molecule yttrium monoxide (YO), the researchers have developed a new approach to manipulate internal states and molecular motion with unprecedented precision.

The advance not only redefines the quantum state control available to laser-cooled molecules, but also lays the foundation for future advancements in quantum simulation, precision measurement, and the potential development of a molecular clock.

From Nuisance to Narrowline

This research relies on a unique property of the yttrium monoxide (YO) molecule: the existence of a long-lived excited electronic state. The longer natural lifetime an excited state possesses, the narrower its transition linewidth is. And these extraordinarily narrow features enable unparalleled spectroscopic precision and can be used to cool molecules below currently achievable temperatures.

It is worth noting that although the long-lived excited state in YO offers immense potential, until recently, it had only provided additional challenges. “If anything, I would say this excited state has historically been a nuisance to laser cooling,” says JILA graduate student Kameron Mehling, the paper’s first author. “Its very presence forced us to modify the already complicated photon cycling schemes necessary to cool YO to begin with.”

Nevertheless, the JILA team has finally harnessed the long-lived electronic state in YO, more than a decade after the idea was initially proposed. By precisely addressing the narrow transition with an ultra-stable laser, they were able to slow down the motion of the molecules (cooling them) via the newly addressed excited state.

Molecules can be cooled with laser light by continuously scattering photons — a technique where matter repeatedly absorbs and emits photons over and over, removing energy and entropy in the process. While this technique has become commonplace for atoms, molecules are trickier due to their extra complexity: they rotate, vibrate, and possess close-lying opposite parity states, making it hard to keep the cycle going.

“This excited state has been continuously occupied as a decay pathway within our previously implemented cycling schemes,” Mehling explains. “However, this is the first time that we’re directly exciting it and exploring the resulting physics.”

The team’s results rely on one of the most accurate spectroscopic measurements ever made in a laser-cooled molecule—resolving the narrowline transition frequency to 11 digits of precision. This highlights the potential of narrowline transitions in laser-cooled molecules for future precision experiments and opened the door for laser cooling.

Expanding the Molecular Control Toolbox

To make narrowline laser cooling practical, the team had to address a longstanding challenge: preventing the molecules from leaking out of the cooling cycle. Their solution came from an unexpected but powerful source—an applied electric field.

In YO, certain energy states come in nearly identical pairs of opposite parity—like twins (think Kameron and Kendall Mehling) with mirrored personalities. It might seem subtle, but mixing up the twins opens unwanted photon “communication” channels and jeopardizes the photon cycling scheme. However, by applying a small electric field, the researchers could identify and isolate a single metastable excited state (i.e. twin) which the laser could repeatedly interact with.

“You have to use another tool in the toolbox,” says JILA postdoctoral researcher Simon Scheidegger.

“Usually in atomic experiments, researchers use light and magnetic fields. But for this, we had to bring in electric fields to isolate the states we care about.”

And the amount of electric field needed? Surprisingly small!

“Other molecular experiments might need 10 to 20 kilovolts per centimeter to observe a similar effect” notes Scheidegger. “We apply fields four orders of magnitude smaller, requiring less voltage than what’s in a AA battery.”

Cooling on the Fly

To demonstrate laser cooling, the team prepared a cloud of ultracold YO molecules and let them fall freely under gravity. While the molecules dropped, they were exposed to carefully tuned laser light and their change in temperature was recorded as the laser frequency was varied.

Despite a brief interaction window, the results were clear: the technique cooled the molecules by a small but significant amount. “Currently we’re limited by how many photons we can scatter off the molecules,” says JILA postdoctoral researcher Logan Hillberry. “Nevertheless, at ultralow temperatures, you are fighting for every additional cooling photon.” The fact laser cooling was demonstrated with only a handful of photons per molecule is particularly impressive —a testament to the technique's efficiency!

“This initial laser cooling demonstration proves we can implement a photon cycling scheme on our narrowline transition, however, there is still plenty of work to be done” says Mengjie Chen, another graduate student on the project. “Since our molecular structure is very well understood, we know we could greatly enhance the cooling effect with only a couple more laser tones.” These future upgrades, along with incorporating the narrowline laser cooling scheme while molecules are trapped in an optical potential, would help initialize record phase space densities and reach currently inaccessible temperatures.

How a Narrow Transition Unlocks Broad Applications

These results suggest more than just a technical milestone— it is a “planting the flag” moment, as the team put it. Narrowline transitions have enabled some of our most precise experiments, like atomic clocks and ongoing searches for fundamental physics. Extending that precision to molecules will unlock entirely new physics. Beyond just laser cooling, the team envisions broad applications across quantum simulation and precision measurements —where molecules are suited to outperform laser-cooled atoms due to their strong electric dipoles. “We’ve built the platform. We’ve demonstrated the tools,” says Mehling. “Now the sky’s the limit.”

In a study published in Physical Review X Quantum, a team led by JILA and NIST Fellow and University of Colorado Boulder physics professor Jun Ye has demonstrated—for the first time—narrow-line laser cooling of a molecule. By utilizing a previously unaddressed transition in the diatomic molecule yttrium monoxide (YO), the researchers have developed a new approach to manipulate internal states and molecular motion with unprecedented precision.

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JILA Collaboration Makes Cavity Quantum Electrodynamics into a Team Sport

Steven Burrows — Thu, 27 Nov 2025 19:01:01 +0000

JILA Collaboration Makes Cavity Quantum Electrodynamics into a Team Sport Steven Burrows Thu, 11/27/2025 - 12:01

Bailey Bedford / Freelance Science Communicator

Researchers used laser light to trigger a rapid sequence where atoms absorb and emit photons, shifting between energy states. Each emitted photon, now a different color, bounces through the cavity and drives the next atom’s transition, enabling a rare three-body interaction. Image Credit: Steven Burrows / JILA

Reality is the result of countless interactions. Everything in daily life, from a grain of dust floating in the air to a neuron firing in a brain, is the result of myriads of atoms and other quantum particles interacting.

Often, we get by with ignoring interactions and seeing just the big picture result. However, physicists learn a lot by digging down to the foundation and studying how the interactions between particles play out. In the past, researchers have mostly simplified things by focusing on interactions between two objects at a time—two-body interactions. However, reality isn’t always so simple. Sometimes three or more particles interact in fundamentally different ways than groups of interacting pairs would.

For the past several years, an experimental research group led by JILA Fellow James Thompson and a theoretical research group led by JILA Fellow Ana Maria Rey have been working together to study quantum interactions using cavity quantum electrodynamics (cavity QED)—the science of how light contained in reflective cavities interacts with quantum particles, like individual atoms. Recently, they tackled many-body interactions with a new experiment, described in an article published in the journal Science. In the experiment, they successfully created interactions that require the participation of either three or four atoms to achieve the observed results.

“Nature’s forces act between pairs, but when many particles come together, new interactions can emerge,” says Rey, who is also a National Institute of Standards and Technology (NIST) Fellow and a University of Colorado Boulder Physics professor. “Controlling these multi-body interactions opens the door to faster, richer and more powerful quantum matter.”

The new experiment took their research from looking at situations where all interactions are essentially the result of atoms playing two-player sports to a more complex world where atoms participate in team sports. Instead of two tennis players hitting a ball back and forth, the experiment introduces a baseball team where the ball gets thrown between several players. The change expands their ability to form quantum connections between the players.

“This is a whole new path to generate quantumy-stuff called entanglement that will improve quantum sensors for navigation, atomic clocks and maybe even detect exotic things like dark matter or gravitational waves,” says Thompson, who is also a NIST physicist and University of Colorado Boulder Physics professor.

The experiment used rubidium atoms as the players, and their games—interactions—were carried out by tossing around light. The researchers used cavities as the playing field and supplied around a thousand atoms to form small teams. The researchers controlled the colors of light they sent into the cavity and how different colors of light behaved in the cavity, which helped them set the rules of the game.

The researchers focused on the quantum states of the atoms defined by the movement of the atoms through the chamber. Thanks to conservation laws, the atoms couldn’t just change their speeds and run around the experiment in any random way; to change states, they needed to receive or release exactly the right amount of energy and momentum. The researchers set up the experiment so the only way the atoms could change states was by catching or releasing photons—individual particles of light that carry specific amounts of energy and momentum.

Since the atoms were in a frictionless vacuum, they didn’t stay in place like a pitcher on the mound when they caught or threw a photon. Instead, it was like the atoms were a baseball team forced to play on ice or were astronauts playing the game of catch while floating in the middle of a spacewalk: Every catch and throw gave them a shove.

The quantum nature of the atoms meant they were only stable in certain specific states, and each atom could only catch a photon if there was an appropriate state for it to move into afterward. This allowed the researchers to carefully design interactions by choosing what colors of light were in the cavity.

They focused on moving atoms between two stable states, and they made sure the cavity didn’t contain light that could simply knock an atom between the states with a single caught photon (the resulting interactions would be boring). Instead, they created a playing field where atoms had to coordinate a specific play—string of interactions—to move between stable states. They ensured that each game started and ended with photons whose energy differed by exactly enough to move three atoms between states.

To start the play, the researchers flooded the cavity with light that could push the atoms to an energetic state that they couldn’t stay in for long. Each time an atom caught a photon, it immediately threw out a photon to return to a lower energy state. Sometimes it threw out a photon just like the one it caught and returned to its original state. Other times, it instead tossed out a weaker photon and kept a little bit of its new energy and momentum. The only allowed option was keeping exactly enough to settle into the second stable state.

This released photon was a new color and was free to bounce around the cavity and quickly be caught by another atom. Similar to the first step, catching the light temporarily shoved the second atom to an unstable state before it, in turn, tossed off another photon. Again, the second atom sometimes kept enough energy and momentum to join the first atom in the new state. The process continued with a third atom joining the first two by catching the new photon and throwing out another weakened photon.

To ensure the chain of events, the researchers set up their cavity to encourage the presence of the initial light and the final photons released in this game of catch while being inhospitable to other undesired colors of light. The dynamics of the light in the cavity and the rubidium atoms’ available quantum states meant the whole play had to happen quickly or not at all.

“We build very strict rules in our system that all three processes have to happen at the same time in order for momentum and energy to be conserved,” says Chengyi Luo, the co-lead author of the paper.

The researchers confirmed the atoms moved between states following the prescribed three-body interactions, and they went a step further. They illustrated the adaptability of the approach by increasing the amount of energy and momentum available to fuel four-body interactions, adding another player to each game of catch. Their observations showed the atoms teaming up into a smoothly running machine and moving in groups of four to the new state.

These demonstrations are just the first steps in exploring many-body interactions with this approach.

“There are a lot of things people need to figure out about how we're going to explore these multibody interactions to make them useful,” Rey, says. “We just saw them, but there are a lot of new behaviors and capabilities to be explored. For example, we think they can be used to emulate exotic superconductors where four electrons team up instead of two electrons like in normal superconductor, producing a new kind of supercurrent that may contribute to high temperature superconductors.”

In the future, experiments should be able to use different quantum states, induce interactions between even larger numbers of particles and make the interactions do practical work. The ability to involve more particles in each interaction provides a new set of tools for researchers. As the technique is explored and refined, it has potential applications in a variety of areas including quantum simulation, quantum computing and quantum sensing.

“I think it's interesting that there's this new way to change the quality of the communication that can happen between all these atoms,” Thompson says. “You really fundamentally change what that communication looks like. It's just an open physics question, like, ‘Well, how good can it be?’ and going further ‘Can we build new quantum states to simulate and explore the universe around us?’”

For the past several years, an experimental research group led by JILA Fellow James Thompson and a theoretical research group led by JILA Fellow Ana Maria Rey have been working together to study quantum interactions using cavity quantum electrodynamics (cavity QED)—the science of how light contained in reflective cavities interacts with quantum particles, like individual atoms. Recently, they tackled many-body interactions with a new experiment, described in an article published in the journal Science. In the experiment, they successfully created interactions that require the participation of either three or four atoms to achieve the observed results.

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Resonant Frequencies: Playing the Edge of Light with a 3-micron Baton

Steven Burrows — Mon, 03 Nov 2025 18:28:05 +0000

Resonant Frequencies: Playing the Edge of Light with a 3-micron Baton Steven Burrows Mon, 11/03/2025 - 11:28

Kenna Hughes-Castleberry / JILA Science Communicator

An ultrastable, scalable and repeatable method for generating soft X-ray beams using a custom-built 3-micron ultrafast laser that is focused into an anti-resonant hollow-core fiber. Image credit: Steven Burrows / JILA

Producing coherent (or laser like) soft X-ray beams in a lab-scale setup represents a many decades-long challenge. Scientists in physics, chemistry, and materials science can use soft X-ray light to study the nanoscale properties of materials and biological systems, to capture behaviors that cannot be seen using visible or even ultraviolet light. But here’s the catch: soft X-rays are notoriously hard to make. To get them, most researchers must travel to large, government-funded synchrotrons—billion-dollar machines, that have limited access and stability. These trips are often rushed, competitive, and only available a few times a year.

Now, a team led by JILA Fellows and ��Ҵ�ý�ƽ�� professors Margaret Murnane and Henry Kapteyn has made a significant advance to make soft X-rays more accessible: with their research group, they have developed an ultrastable, scalable and repeatable method for generating soft X-ray beams using a custom-built 3-micron ultrafast laser that is focused into an anti-resonant hollow-core fiber. This breakthrough, detailed in a paper recently published in APL Photonics, builds on well over a decade of laser development. It presents a technological and experimental advance in high-harmonic generation (HHG), the nonlinear optical process by which high-frequency light is created from lower-frequency driving lasers. The team’s past breakthroughs had shown that the key to generating bright coherent soft X-ray beams was to use mid-infrared (2 – 4 µm) driving lasers focused into a waveguide filled with high-pressure gas. However, no good robust drive lasers existed. In this new breakthrough, the team made giant leaps in transitioning the technique from a heroic optics experiment towards a reliable, applications-oriented light source.

“We wanted to make a coherent X-ray source that doesn’t require a team of optics experts to babysit—something that could find applications in labs across various scientific disciplines and industries,” says JILA research associate Drew Morrill, one of the lead scientists on the project and the paper’s co-first author.
Drew and the team have made a huge step forward by creating bright, ultrastable, coherent soft X-ray beams. In the future, they can enable higher-resolution microscopes that can work in a stroboscopic mode—for example, by capturing nanoscale processes in nanoelectronic, quantum, energy and biological systems, making it possible to understand and optimize them.

A Decade in the Making

Developing JILA’s compact soft X-ray source took over ten years of effort—refining a homebuilt 3-micron wavelength ultrafast laser system when no commercial options existed. From the beginning, the goal was ambitious: to build a mid-infrared laser that was not only powerful and ultrafast but stable enough to operate for entire days without interruption.

To reach that level of performance, the team had to learn how to build fiber lasers from the ground up. That meant mastering delicate tasks like fiber splicing, amplifier construction, and dispersion balancing—adapting technologies initially designed for telecommunications into a new realm of nonlinear fiber optic to seed high power lasers.

One key laser advance came during the early months of the COVID-19 pandemic when the team collaborated with ��Ҵ�ý�ƽ�� Engineering and Physics Professor Scott Diddams. “Scott’s group gave us a roadmap—parts lists, layout guidance, and design principles,” says JILA research scientist Michaël Hemmer, one of the paper’s lead authors. “Then we built it ourselves. The pulses provided by this front-end are outstandingly stable and really the cornerstone of the laser system. These pulses are then amplified using a home-built ytterbium-doped crystal amplifier, providing the high energy needed for HHG while maintaining a clean, controlled beam.”

“The cryogenic ytterbium amplifier is also a second key building block of the system, but it can only run reliably because the front-end is exceptionally reliable; otherwise, it would destroy itself all the time,” notes Hemmer.

Another key contributor was European physicist Dr. Gunnar Arisholm, who shared advanced simulation code that helped the team model complex optical interactions in nonlinear crystals.

“It saved us months of trial and error,” says Hemmer. “He helped train Drew to use the code, which was instrumental in getting the final version running.”

And finally, the key advance was to use optimized waveguides for efficiently converting the laser light into coherent soft X-ray beams.

The line of the first three OPA's that amplify the 3-micron beam. The green light is the parasitic second harmonic light of the 1-micron pump, and the red light is the sum frequency of the 1-micron pump and the 1.5-micron signal. Credit: Gabriella “Gabi” Seifert / JILA

Building and Testing a New Instrument

After designing and re-designing the laser system featuring a fiber-laser-seeded optical parametric chirped-pulse amplifier (OPCPA), the team was finally able to deliver 3 µm wavelength laser pulses with exceptional power and stability. To upconvert this laser light into soft X-rays, the laser pulses are guided through an engineered anti-resonant hollow-core fiber (ARHCF) filled with high-pressure noble gas. Working as a “conductor” for the light, the fiber acts as a waveguide and a container for the interaction medium, allowing the laser and the emitted soft X-rays to travel in phase and interfere constructively over large lengths—opening the door to a new regime of compact, high-brightness sources.

“The laser light travels through the fiber, ionizes the gas, and emits harmonics—overtones of light—far above the frequency of the original beam,” explains JILA graduate student and co-first author Will Hettel.

This process, known as high-harmonic generation (HHG), converts mid-infrared laser pulses into coherent soft X-ray light—similar to how plucking a violin string produces overtones from a single note.

To support this process, the team, with the help of JILA instrument maker and co-author James Uhrich, engineered a precision target system with a modular design: a chassis that allows rapid reconfiguration for different gases and geometries, streamlining the experimental workflow.

“We designed a setup where we can swap out fiber cartridges with micron-level precision,” says Hettel. “It stays aligned even under 10 atmospheres of pressure.”

In terms of output, the system generates soft X-ray photons at energies exceeding 280 eV, reaching the carbon K-edge—a crucial spectral region for biological and materials science applications.

From their design, the researchers found that the setup can run at kilohertz-level repetition rates with continuous, stable beam output for several hours or longer with minimal fluctuation. The system is also rather robust, showing no signs of optical damage even after months of operation. This level of durability is essential for research workflows that demand high uptime and minimal maintenance.

“This isn’t a one-off result,” said Hemmer. “We can run it for days. The beam doesn’t drift. The power doesn’t degrade. That makes it incredibly useful for real experiments.”

Simulating a Symphony

While the laser system was being constructed, another crucial component of the project unfolded in parallel: advanced simulations. To better understand and optimize the HHG process, JILA graduate student Ben Shearer helped develop a fast and flexible numerical model.

“Simulations like this normally take days or weeks to run,” Shearer explains. “We created a version that runs in hours or even minutes—without sacrificing too much of the physics.”

His code, based on a parameterized version of the strong-field approximation, allowed the team to virtually test a wide range of laser pulse durations, energies, and gas conditions before trying them in the lab.

“Ben’s work gave us a cheat sheet,” notes Hemmer. “We could avoid dead ends and prioritize ideas that had a real shot at working.”

These simulations also laid the groundwork for future upgrades, such as transitioning from argon to helium to achieve even higher photon energies.

“If you want to go to the absolute highest energy of high harmonic generation, you need to ionize helium,” says JILA graduate student Gabriella “Gabi” Seifert. “We're getting there; it’s just taking it one step at a time.”

Helium’s higher ionization potential allows stronger driving fields without over-ionizing the medium—a key requirement for pushing HHG to higher energy regimes.

A view of the argon gas cell that the laser is beamed through to produce HHG, showing the fifth harmonic (yellow) and seventh harmonic (blue). Credit: Drew Morrill and Grzegorz Golba / JILA

A World of Possibilities

By building a stable, coherent soft X-ray source that fits on a lab bench, the team has opened the door for broader scientific access to a tool that once required massive infrastructure with limited access.

“We’re really just scratching the surface of what this source can enable,” says Morrill. “With this kind of stability and control, we can start to ask questions that were previously only addressable at synchrotron or free-electron laser facilities, and even go beyond what was possible before.”

Potential applications include high-resolution soft X-ray microscopy of carbon-rich biological material—opening up the possibility of live cell imaging without the need to add light-emitting fluorescent molecules or without the need to freeze the sample.

“This spectral regime is well suited for high-resolution biological imaging,” says JILA graduate student Clay Klein
Other uses lie in probing advanced magnetic materials, such as those explored for ultra-low-energy computing or data storage technologies based on electron spin.

“There’s a long history of new light sources unlocking unexpected science,” said Morrill. “We’re excited to see where this one leads.”

This research was published in APL Photonics.

A team led by JILA Fellows and ��Ҵ�ý�ƽ�� professors Margaret Murnane and Henry Kapteyn has made a significant advance to make soft X-rays more accessible: with their research group, they have developed an ultrastable, scalable and repeatable method for generating soft X-ray beams using a custom-built 3-micron ultrafast laser that is focused into an anti-resonant hollow-core fiber.

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Entangled Time: Pushing Atomic Clocks Beyond the Standard Quantum Limit

Steven Burrows — Thu, 23 Oct 2025 16:24:51 +0000

Entangled Time: Pushing Atomic Clocks Beyond the Standard Quantum Limit Steven Burrows Thu, 10/23/2025 - 10:24

Steven Burrows / JILA Science Communications Manager

Artistic representation of an atomic clock breaking the Standard Quantum Limit. Image credit: Steven Burrows / JILA

Imagine you're trying to keep time by listening to a room full of people clapping. If everyone claps randomly, it’s hard to tell the rhythm. But if they clap in sync, the beat becomes clear and steady. Now imagine you could gently guide them to clap more in unison—not perfectly, but just enough to reduce the noise. That’s what JILA researchers have done with atoms in a clock.

Their results, published in Physical Review Letters, show that it’s possible to go beyond what’s known as the Standard Quantum Limit (SQL)—a fundamental barrier in quantum measurements—by using a technique called spin squeezing. This work could help improve everything from GPS systems to tests of gravity and the nature of the universe.

What Limits a Clock’s Precision?

Atomic clocks are among the most precise instruments ever built. They work by measuring the frequency of light that causes atoms to jump between energy levels. These transitions are incredibly stable, making them ideal for keeping time. But there’s a catch. Each atom behaves independently, and their random quantum behavior adds noise to the measurement. This randomness is what defines the Standard Quantum Limit. It’s like trying to hear a single beat in a noisy crowd.

To reduce this noise, scientists often increase the number of atoms. The more atoms you measure, the better your estimate—kind of like averaging more coin flips to get closer to 50/50. But packing too many atoms together causes them to interact in ways that shift the clock frequency, introducing new errors. So instead of adding more atoms, the JILA team tried something different: they made the atoms entangled.

Entanglement is a quantum connection between particles. When atoms are entangled, their random quantum behavior becomes linked—even if they’re not touching. In this experiment, the researchers used entanglement to make the atoms behave more like a team, reducing the noise in their collective signal.

This approach allows the clock to beat the SQL, achieving better precision without needing more atoms. It’s a clever way to get more information out of the same number of particles.

Entanglement through Nondemolition Measurement

To entangle the atoms, researchers Dr. Yang Yang, Maya Miklos, and their lab mates used a method called quantum nondemolition (QND) measurement. This means they could measure the atoms without disturbing them too much, like checking the temperature of soup without taking the lid off.

They trapped about 30,000 strontium atoms in a grid of laser light called a two-dimensional optical lattice. This setup holds the atoms in place and keeps them cold—less than a millionth of a degree above absolute zero. Cold atoms move less, which helps maintain their coherence and reduces unwanted interactions.

The atoms were placed inside an optical cavity, which bounces light back and forth to enhance its interaction with the atoms. By shining a special probe light into the cavity, the researchers could gently measure the atoms’ collective spin—a property related to their energy state—without collapsing their individual quantum states. The team also used a technique called spin echo to cancel out unwanted shifts caused by the probe light. This helped preserve the delicate quantum state of the atoms during the measurement.

This process “squeezes” the uncertainty in one direction, reducing the noise in the measurement. It’s like squeezing a balloon: the uncertainty gets smaller in one direction but bigger in another. For clocks, this trade-off is worth it because it makes the timing signal more precise when one measures along the squeezed direction.

Putting the Squeezed Clock to the Test

To see if their entangled clock really worked better, the researchers compared two groups of atoms in a “synchronous comparison” between two atomic ensembles. By comparing two clocks at the same time, they could cancel out common sources of noise—like fluctuations in the laser used to probe the atoms. This allowed them to isolate the improvement due to spin squeezing: they can compare the case where both samples are regular, unentangled atoms (called a coherent spin state, or CSS), to where each sample is prepared in a spin-squeezed state (SSS) to see the improved stability from spin squeezing.

They studied how precisely the clock comparison signal could be measured over time. The spin-squeezed clock showed a 2.0 decibel improvement beyond the Standard Quantum Limit. That might not sound like much, but in the world of precision measurement, it’s a significant step forward. They found that the spin-squeezed clock not only beat the SQL but also showed a 3.3 dB improvement over the unentangled clock. This confirms that the entanglement was not just a theoretical benefit—it made a real difference in the clock’s performance.

Over a 43-minute test, the clock reached a fractional frequency uncertainty of 1.1 × 10⁻¹⁸. That means it could detect a change in time as small as one second over the age of the universe. This is the most precise entanglement-enhanced clock ever demonstrated, proving that such entanglement could in the future help make the world’s best clocks even more precise.

Why Does This Matter?

This research is part of a broader effort at JILA to explore how quantum physics can improve measurement tools. JILA Fellows Adam Kaufman and James Thompson are also exploring the use of entanglement for better measurement precision. Atomic clocks are already used in GPS satellites, telecommunications, and tests of fundamental physics. Making them even more precise opens new possibilities. A key challenge is to demonstrate genuine quantum advantage where an entangled clock can reach a performance level superior to the best clock today.

For example, ultra-precise clocks can measure tiny differences in gravity across short distances. This could help scientists study how gravity affects quantum systems or even searches for new physics beyond Einstein’s theories.

The techniques developed in this study—like spin squeezing and QND measurements—could also be used in other quantum technologies, such as sensors and quantum computers. These tools rely on the same principles of coherence and entanglement to perform tasks that classical systems can’t.

Looking ahead, the team hopes to improve their system by using three-dimensional optical lattices, which offer even better control over the atoms. They’re also exploring new ways to amplify signals using time-reversal techniques and quantum optimization algorithms.

There is also growing interest in using entangled clocks to probe the interface between quantum mechanics and gravity. Recent studies together with JILA Fellow Ana Maria Rey and external collaborators at University of Innsbruck have explored how mass-energy equivalence and gravitational gradients affect entangled states, raising fundamental questions about the nature of time and space.

A New Chapter in Quantum Timekeeping

By using entanglement to reduce quantum noise, JILA researchers have taken a meaningful step toward the next generation of atomic clocks. Their work shows that it’s possible to go beyond traditional limits by carefully engineering both the quantum states of atoms and the tools used to measure them.

As clocks become more precise, they also become more sensitive to the world around them. This opens the door to new experiments in gravity, quantum mechanics, and the structure of space-time itself.

In the end, this research isn’t just about keeping better time—it’s about using time to explore the microscopic and macroscopic side of the universe in new ways.

This research is supported by the US Department of Energy, Office of Science, National Quantum Information, Science Research Centers, Quantum Systems Accelerator; National Science Foundation; V. Bush Fellowship; JILA Physics Frontier Center; and the National Institute of Standards and Technology.

In a new study, researchers led by JILA and NIST Fellow Jun Ye have shown how to make atomic clocks even more precise by leveraging entanglement. This allows the atoms to “tick” more in sync, reducing the randomness that usually limits how precisely we can measure time.

Their results show that it’s possible to go beyond what’s known as the Standard Quantum Limit (SQL)—a fundamental barrier in quantum measurements—by using a technique called spin squeezing. This work could help improve everything from GPS systems to tests of gravity and the nature of the universe.

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Building the quantum workforce of the future: A new study seeks the way

Steven Burrows — Wed, 08 Oct 2025 17:28:39 +0000

Building the quantum workforce of the future: A new study seeks the way Steven Burrows Wed, 10/08/2025 - 11:28

Daniel Strain / ��Ҵ�ý�ƽ�� Strategic Communications

In recent years, quantum technology companies have begun to pop up across the United States. These companies design technologies that tap into some of the unique properties of very small things like atoms and electrons. Such technologies include “quantum computers” that could one day discover previously unknown medications, or sensors that can detect signs of illness in a single puff of breath. But the growth of the industry also raises a major question, said physicist Heather Lewandowski, one of the project leads: How can the nation better prepare students to enter this uncharted industry?

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Tailoring Record-Breaking Laser Stability for Coordinating Precise Atomic Dances

Steven Burrows — Wed, 01 Oct 2025 16:22:35 +0000

Tailoring Record-Breaking Laser Stability for Coordinating Precise Atomic Dances Steven Burrows Wed, 10/01/2025 - 10:22

Bailey Bedford / Freelance Science Communicator

3D optical lattice clock platform for highfidelity quantum state engineering.

Image Credit: Steven Burrows / JILA

Light is incredibly useful in daily life. We use light to see objects and determine details about them. Light is similarly valuable in probing the quantum world. It is often critical for both observing quantum objects and interacting with them.

When scientists need to precisely control atoms or molecules, light is often the only tool for the job. Selecting the correct frequency—color—of laser light and projecting it in the right configuration allows scientists to detect, trap, and even manipulate individual quantum particles.

However, keeping a laser stable at the right frequency is challenging. Even the most stable lasers randomly shift to slightly different frequencies and experience noise—random spurts of different frequencies similar to static on a radio signal. This frequency noise is currently one of the main limitations of lasers in many experiments. As researchers improve lasers, the improvements reliably produce better experiments and technologies, including more precise atomic clocks and quantum computers that experience fewer errors.

“Every quantum scientist dreams of having a laser that can keep driving quantum systems without introducing errors,” says Lingfeng Yan, a graduate student at JILA.

A team of researchers, led by JILA and National Institute of Standards and Technology Fellow and University of Colorado Boulder Physics professor Jun Ye, took on the challenge of tailoring a laser system to an unprecedented level of stability and showing the improvements it could deliver for practical applications. Achieving this new level of stability required them to make multiple lasers work together.

In an article published in the journal Physical Review X on August 26, 2025, they described their laser setup and showing the improvements it could deliver for practical applications. They showed that the laser delivered practical advantages by putting many neutral atoms through their paces working as a qubit—the basic building block of a quantum computer—and achieving an unprecedented low error rate for the particular design of qubit used.

The bespoke laser was needed because lasers aren’t all equal. Even with the best available designs, lasers of some colors are more stable than others in particular situations, and it’s impossible for any particular laser to do every job.

Fortunately, researchers can impart the stability of one laser onto another. It is like a dance teacher who has one student who is perfect at keeping their timing no matter how long the dance and another who is great at performing the necessary steps but frequently speeds up or slows down randomly. The teacher pairs them up, and whenever they notice the student messing up, they remind them to follow the lead of their partner. Properly directed, the group exceeds the performance of the individuals.

The group has access to a laser that can stay stable for extended periods—a prima ballerina. The researchers decided to test how well they could do at transferring its stability to a less stable dancer—specifically a laser compatible with altering the quantum states of strontium atoms. Such lasers are used to manipulate strontium in certain atomic clocks and quantum computers.

The lab’s stable dancer was a laser cavity made from a silicon crystal. The crystal’s rigidity makes it very stable over extended periods of time, but it must be kept at frigid temperatures to not be negatively impacted by temperature fluctuations.

“It is one of the best lasers in the world,” says Yan, who is the first author of the paper. “It provides an excellent long-term stability, but it's a specialized cavity.”

The specialized design means it is expensive and works for just a specific set frequency. So, to get similar performance at other frequencies, the team needed to become dance instructors and get other lasers to follow the silicon cavity’s lead over the long term.

Unfortunately, you can’t just yell dance instructions to a laser. The researchers had to use a specialized tool, called a frequency comb, to coordinate their lasers. A frequency comb is a device that, instead of producing a single laser beam, produces many precise, evenly spaced frequencies of light. The regular spacing makes frequency combs ruler-like tools for comparing different lasers and maintaining the frequency spacing between them.

However, even with the silicon cavity and frequency comb in the loop, the final beam would still experience high-frequency noise that would impair its use. This is largely because even the silicon cavity contributes a little noise, including some introduced by vibrations from the necessary cooling equipment.

To tamp down this residual noise, the researchers added another cavity to the dance: a simpler cavity that operated at the same laser frequency used to manipulate the strontium atoms. The second cavity is less stable over long times but doesn’t need to be cooled and therefore doesn’t experience the remaining troublesome noise over shorter periods. This second cavity handled suppressing their high-frequency noise issues while letting the silicon cavity steer the frequency over the long term.

The team carefully coordinated the appropriate set of correction procedures and technological connections between the two cavities, the optical frequency comb and the final laser, but that was just the beginning. The group still needed to confirm if their laser setup worked as intended. Was the meticulously tailored custom laser actually stable and could it deliver improved results?

The team created a test for themselves: Shining the laser at strontium atoms. The atoms’ sensitivity to specific light frequencies made them a precision tool for checking exactly how the laser was behaving. The researchers essentially turned the atoms into a tool for measuring the laser-frequency noise of the laser.

In the test, the strontium atoms reacted to subtle fluctuations in the light and could catch details that are otherwise easily missed. For example, during one test, they discovered an unexpected spike in the noise despite the laser seeming to run correctly. They discovered the noise was because a device designed to prevent the silicon cavity from vibrating had accidentally been turned off.

“What we trust most are measurements of the atomic response,” says Max Frankel, a graduate student at JILA and a co-author of the paper. “Atomic measurement should have the final word on our laser frequency noise model.”

Their test confirmed that their new setup delivered the improved performance they had predicted. Then, they moved on to demonstrating the practical advantage of all their effort by using the laser to make the atoms perform as qubits in a standardized test.

Using the stabilized laser, they performed strings of many gates—the basic operations of quantum computers—on each of 3000 qubits. They used gates that essentially signal an atom’s quantum state to spin around to various positions, which physicists call performing state rotations. Then, the researchers performed the gate that should reverse the whole string of operations. As long as noise didn’t interfere, the laser guided all the qubits through the set of steps to the same final position. By analyzing how well the qubits returned to their initial state over many runs, the researchers determined how reliably the laser executed the gates on average. They established a new record for the fidelity achieved using a laser to optically manipulate neutral atoms to perform state rotations.

The results of their test also match well with their model of the laser noise, which they say suggest that further laser improvements will likely deliver even better results. The team says that other researchers should be able to use the same techniques to tailor lasers with different frequencies to have similar refined stability.

“Lasers are central to manipulating quantum systems, which are very sensitive to imperfections, so improving lasers benefits scientists and engineers all over the world.” says Stefan Lannig, a JILA postdoctoral researcher and co-author of the paper. “To benefit from many ideas put forward by modern science, we need to enhance our control over intricate quantum systems, which requires first improving our tools.”

Jun Ye's research group has developed a groundbreaking laser system with record-breaking stability, crucial for advancing quantum technologies. By combining a highly stable silicon cavity laser with a frequency comb and a secondary cavity tuned for strontium atoms, the researchers created a laser capable of manipulating quantum states with unprecedented precision. Their system significantly reduces frequency noise, a major hurdle in quantum experiments, and demonstrated its effectiveness by achieving a new fidelity record in quantum gate operations on 3000 neutral atom qubits. This innovation paves the way for more accurate atomic clocks and scalable quantum computing.

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