Researchers approach the Mount Blue Sky Observatory.

Laura Sinclar, a NIST researcher, inspects a frequency comb laser.

Researchers set up equipment inside the Mount Blue Sky Observatory.

Quantum physicists measure whether time moves faster on a mountaintop

CU Boulder researchers recently found their way atop Mount Blue Sky, one of Colorado’s famous 14ers, with the same ambitious goal: to advance quantum physics. 

Over the last three summers, scientists from CU Boulder, the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration (NOAA) are using an optical atomic clock to measure how time changes with altitude. The project is funded by the U.S. National Science Foundation (NSF) and NIST. 

Science in unforgiving conditions 

The effort centers on determining whether time moves measurably faster farther from Earth’s center. The work also serves as a test of whether sophisticated quantum technologies can operate reliably outside controlled laboratory environments. 

The research could have implications far beyond fundamental physics, said Scott Diddams, professor of electrical, computer and energy engineering (ECEE). Optical atomic clocks measure time by tracking energy transitions in atoms with extraordinary precision. Making these devices smaller and more robust could transform applications ranging from volcano monitoring to spacecraft navigation. 

“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,” Diddams said. 

The project highlights Colorado’s leadership in quantum science. Since 2023, Elevate Quantum—a coalition of 120 organizations in the Mountain West, with CU Boulder as a prime partner—has secured more than $120 million in federal and state funding to grow the region’s quantum economy. 

From mountains to atoms 

This particular experiment takes place inside the former Meyer-Womble Observatory on Mount Blue Sky. At the heart of the operation is an optical atomic clock containing about 300 ytterbium atoms. Scientists cool these atoms to extremely low temperatures and use lasers to make them “tick” nearly a quadrillion times per second. Researchers then measure those oscillations using a frequency comb laser, a technology pioneered at NIST and JILA, the joint institute between CU Boulder and NIST. 

The research team, known as the Relativistic Redshift Peak to Plains (R2P2) collaboration, is testing one of Einstein’s most famous predictions. According to general relativity, gravity slows the passage of time. Because gravity weakens with distance from Earth’s center, time should move slightly faster at higher elevations. 

To test that idea, researchers are comparing a clock on Mount Blue Sky with an identical clock in Boulder, more than 8,500 feet below. The predicted difference is tiny—about 25 nanoseconds over a day. 

“There’s not a lot of precedent for making measurements at the level that we need to make them,” said Andrew Ludlow, a NIST scientist, ECEE adjoint professor and CU Boulder alumnus. “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.” 

Time on our side 

The experiment could open new possibilities for geodesy, the science of measuring Earth’s shape and gravitational features, which could yield potentially life-saving results. 

“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 NOAA physicist Derek van Westrum. “In principle, it could be used to warn people that something might be happening.” 

For Diddams and Ludlow, the effort represents decades of progress in quantum science and the enduring partnership between NIST and CU Boulder. 

“This work could not be done anywhere else in the U.S.,” Diddams said.