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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 ��ɫ��Ƶ, 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, —a coalition of 120 organizations in the Mountain West, with ��ɫ��Ƶ 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 between ��ɫ��Ƶ 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 ��ɫ��Ƶ 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."