14er science: Quantum physicists measure whether time moves faster on a mountaintop
Story by Daniel Strain; photos by Glenn Asakawa; video by Nico Goda & Jesse Morgan Petersen
The white pickup truck pulls up to a decommissioned space observatory on top of Mount Blue Sky, one of Colorado’s famous “14ers,” mountains that reach more than 14,000 feet high. The scene is stark on this August day: Wind whips over the rocky summit, and, as if in a nod to the peak’s name, not a single cloud can be seen in the sky.
Researchers led by Laura Sinclair, a scientist at the U.S. National Institute of Standards and Technology (NIST), hop out and begin unloading black cases. Before long, a nanny mountain goat wanders over and begins licking the inside of the truck’s wheel well, slurping up precious salts.
The researchers have come to this site, seemingly at the ends of the world, for an unusual purpose: to advance quantum physics.
Over three summers, the researchers will use a device called an optical atomic clock to test a prediction in Albert Einstein’s theory of general relativity—capturing what might be the most precise measurement yet of how time moves faster the farther you get from the center of Earth.
Theadora Triano, a graduate student at the University of Colorado Boulder, is a member of the team. She grew up near Lake Tahoe in California at roughly 6,000 feet, but, even with her mountain upbringing, she gets winded as she carries equipment.
“This is my first time on a 14er,” says Triano, who’s studying electrical engineering. “It’s very rare that you can encounter this drastic a difference in altitude so close to a major research institution.”
The research is about much more than just altitude and time, said Scott Diddams, professor in the Department of Electrical, Computer and Energy Engineering (ECEE) at CU Boulder.
The campaign on top of Mount Blue Sky will put these next-generation atomic clocks to the test in a way that’s never been done before. These devices keep track of time with remarkable precision and accuracy by measuring the energy levels of atoms. By making these clocks smaller and more reliable, the technology could revolutionize everything from predicting when volcanoes are about to erupt to navigating spacecraft to other planets.
“This is unprecedented,” Diddams said. “When we built the first optical clocks 25 years ago, we never would have dreamed such a combination of performance and remote operation would be possible.”
The project is funded by the U.S. National Science Foundation and NIST and is a partnership between CU Boulder, NIST and the U.S. National Oceanic and Atmospheric Administration (NOAA).
The team is tackling one of the biggest challenges in physics today: After years of scientific advancement, can researchers take quantum technology out of the lab and into the real and unforgiving world?
“It’s pretty exciting to be part of something so big,” said Sinclair, an adjoint professor at ECEE who earned her doctorate in physics in 2011 from CU Boulder. “We’ve talked about it for a lot of years…but we’re really doing it.”
From mountains to atoms
That history-making experiment is taking place inside a quiet building with a domed roof on the summit of Mount Blue Sky.
The University of Denver ran the Womble Observatory from 1996 to 2018. Today, it’s bursting with people and electronics—wires, computer monitors and laser systems that bend light through networks of lenses and mirrors.
The entire setup, however, hinges on something much, much smaller.
“This is the rack that holds our atomic system,” says Eric Swiler, a graduate student in physics at CU Boulder, pointing to a file cabinet-sized stack of equipment.
There, beneath layers of metal shielding no bigger than a grapefruit, lies the heart of the optical atomic clock: about 300 ytterbium atoms.
Small is the watchword in quantum physics. In simple terms, the field is the study of objects like atoms and electrons—things so tiny that they don’t behave like the much larger world humans are used to. Here, light exists as both a particle and wave, electrons tunnel through solid matter and, in the famous thought experiment of Schrödinger’s cat, household pets can be both alive and dead.
As Sinclair put it: “When you go to a conference and people are talking about quantum physics and quantum information, they always start their talks with, ‘Well, it seems very counterintuitive, but…’”
The Solvay Conference of 1927, which focused on electrons and photons, was one of the most formative moments in establishing the theory of quantum mechanics. Held in Norway, the event brought together such luminaries as Neils Bohr, Marie Curie, Albert Einstein and Erwin Schrödinger.
Counterintuitive, yes, but technologies based on this world of small things hold the potential to transform human lives in Colorado and beyond. Since 2023, Elevate Quantum—a coalition of 120 organizations in the Mountain West, with CU Boulder as a prime partner—secured more than $120 million in federal and state funding to grow the region’s quantum economy.
Optical atomic clocks have become central to that burgeoning industry.
Scientists in Colorado, including at JILA, a joint research institute between CU Boulder and NIST, have pioneered the design of these devices for more than two decades. The ytterbium clock on Mount Blue Sky was developed by Andrew Ludlow, a scientist at NIST, and his colleagues.
To make one of these devices, researchers first cool a cloud of atoms down to incredibly cold temperatures. They then use lasers to make the atoms “tick.” The lasers knock electrons orbiting those atoms from a low energy level to a higher energy level—over and over again. It’s a bit like pushing on the pendulum of a grandfather clock to get it swinging.
Atoms also tick fast, or nearly a quadrillion times per second. The ytterbium clocks can measure that ticking out to 18 digits. By slicing time into smaller and smaller units, the researchers say, they can track time with much greater precision than existing clocks.
Physicists read out that ticking using a specialized laser known as a frequency comb, which scientists at NIST and JILA invented in the late 1990s. Unlike a traditional laser, which emits light in only one color, frequency combs send out millions of colors of light, all at the same time.
“There’s not a lot of precedent for making measurements at the level that we need to make them,” said Ludlow, who earned his doctorate from CU Boulder in 2008 and is a professor adjoint at ECEE. “Then we're adding into the mix that we're not doing it in the lab. One of the clocks has to be up on a mountain top exposed to some harsher conditions."
Scott Diddams
As a postdoctoral researcher at JILA in the late 1990s, Diddams was part of the team that developed the first frequency comb lasers. (Credit: CU Boulder College of Engineering and Applied Science)
Andrew Ludlow
During his own time as a graduate student at JILA in the 2000s, Ludlow helped to develop the first optical lattice clocks, an advanced type of atomic clocks. (Credit: NIST)
Laura Sinclair
In 2023, Sinclair led a team that used frequency comb lasers to beam a time signal between two Hawaiian Islands nearly 100 miles apart. (Credit: NIST)
Bending space and time
That accuracy is allowing the team to pursue a phenomenon that Einstein proposed more than 100 years ago.
Skyler Weight, a graduate student in physics at CU Boulder, is member of the research project, called the Relativistic Redshift Peak to Plains collaboration, or R2P2 for short. He imagines a scenario: Say you and a friend synchronize your watches. Then you hike to the top of Mount Blue Sky while your friend stays in Boulder, Colorado.
“You call your friend and say, ‘Hey, what time is it for you?’” Weight said. “Your friend says it’s 4:31, and you look at your watch, and it says it’s 4:32…Why is it telling us a different time?”
Researchers install a Starlink satellite dish on the summit of Mount Blue Sky.
Theadora Triano working in the Womble Observatory.
The answer, according to the predictions of Einstein, is that the effect of gravity can stretch time, almost like tugging on saltwater taffy. The stronger gravity gets, the slower time moves, albeit by an incredibly small amount. On Earth, gravity should be at its weakest (and time should move the fastest) the farther you get from the gravitational pull from the planet’s central mass—like on top of a 14,000-foot mountain.
To measure the strength of this effect, the researchers will first use Ludlow’s clock to record time from the summit of Mount Blue Sky. Then, through a complicated game of engineering telephone, they’ll compare that signal to the ticking of an identical clock in Boulder, more than 8,500 feet lower.
According to theory, the actual difference in ticking between the two clocks will be far less than the one second in Weight’s example, or on the order of about 25 nanoseconds over a day.
Optical atomic clocks could allow the scientists to measure that difference with a precision never seen before—revealing whether Einstein’s predictions hold true, or if his theory needs updating.
If the team can make the measurements work, the possibilities could be immense, said Derek van Westrum, a physicist at NOAA. He’s part of a team that uses more traditional devices called gravitometers to, among other things, measure the effective height of geologic features like mountains, a field known as geodesy.
Atomic clocks could, one day, measure those kinds of gravitational intricacies much more reliably and in real time. That would help emergency managers across the country to, for example, better predict how rivers might flood during a storm. Such devices could also detect magma rushing far below the Earth’s surface.
“If you put a clock on the side of a volcano, and it starts to run…more slowly than it used to, that might be an indication that magma is coming up,” said van Westrum, who earned his doctorate in physics from CU Boulder in 1998. “In principle, it could be used to warn people that something might be happening.”
Syncing up clocks
To compare the ticking of two atomic clocks, researchers will beam lasers over the air between Mount Blue Sky and Thornton, and by fiberoptic cables between Thornton and Boulder. (Credit: Hanna Nordwall/ CU Boulder College of Engineering and Applied Science)
On the second floor of the Womble Observatory on the summit of Mount Blue Sky, a monitor displays a fuzzy, gray image with a blinking light at its center. The image on the monitor is a zoomed-in view from a laser emitter perched on the outside of the observatory. And that blinking light comes from a beacon on an Adams 12 Five Star Schools administration building in Thornton, Colorado, more than 50 miles away.
NIST’s Laura Sinclair explained it’s one thing to measure time on the summit of a mountain. It’s another to capture that measurement and compare it to the time from a second clock. To do that, the R2P2 team uses a series of frequency comb lasers.
First, Sinclair and her colleagues will synchronize one frequency comb laser to the ticking of the Mount Blue Sky atomic clock. They’ll beam these laser pulses, which pose no risks to people, to the beacon in Thornton. From there, the time signal will travel another 20 miles or so through fiber-optic cables. It will reach NIST’s campus in Boulder where an identical atomic clock will also be keeping time. Researchers can then compare how time measurements from the two clocks line up—or, if Albert Einstein had it right, how different their readings are.
In 2023, Sinclair’s team showed that such a transfer is possible by beaming a similar signal between two Hawaiian Islands nearly 100 miles apart.
“We’re getting to probe the ground rules of the universe,” she said.
A Colorado legacy
This year, the group is mainly testing out its technology, in part to see if it can survive the extremes of a 14er. As the summer drew to a close, the researchers successfully linked the clock on Mount Blue Sky to its twin in Boulder. Next year, they will make a much more precise comparison between how fast the clocks tick.
For Diddams and Ludlow, the project represents a culmination of decades of research in quantum physics. Diddams was a postdoc on the team at JILA that built the world’s first frequency comb laser. The advancement earned his mentor, Jan Hall, a Nobel Prize in Physics in 2005. Ludlow similarly helped to develop the first optical lattice clocks, a type of optical atomic clocks, as a student at JILA in the 2000s.
They also say the effort is a testament to the decades-spanning partnership between NIST and CU Boulder.
“This work could not be done anywhere else in the U.S.,” Diddams said.
Weight sees the project as a uniquely Colorado experience.
“One minute, I could be in the lab looking at a computer screen, and the next I go up to this beautiful landscape. I'm on top of a mountain,” he said. “Not only am I in a place that I love, but I'm doing science in a place that I love.”
Learn more about quantum at CU Boulder
