Published: Nov. 1, 2023 By

Imagine being able to measure tiny changes in the flow of time caused by Earth’s gravity with atomic clocks atop one of Colorado’s iconic peaks above 14,000 feet. 

That could soon be a reality thanks to a $1.9 million grant from the National Science Foundation that will advance geodesy — the study of accurately measuring Earth’s geometric shape, orientation in space and gravity field — through the use of quantum sensors, some of the most precise in the world. 

Scott Diddams, professor in CU Boulder’s Department of Electrical, Computer and Energy Engineering (ECEE), is collaborating on this four-year, multi-agency effort with physicists from the National Institute of Standards and Technology (NIST) and the National Oceanic and Atmospheric Administration (NOAA). To further get students involved, Diddams aims to bring undergraduate and graduate researchers in on the endeavor.  

“Our vision is to take the best quantum science from the lab and translate it out to the world,” said Diddams. “It’s going to be an important activity for the university and field to show how optical clocks can impact the field of geodesy.” 

Albert Einstein’s theory of general relativity states that time evolves more slowly under the influence of gravity known as the gravitational redshift. Essentially, a clock at higher elevations will tick at a faster rate than ones closer to the Earth. 

Diddams and the research group are developing a portable hyper-accurate optical atomic clock, which will be the most advanced quantum sensor of time to operate at such a high elevation. 

Andrew Ludlow, an adjoint professor with ECEE and the NIST physicist building the ytterbium optical clocks used in the project, noted, “if you can measure time extremely well with these atomic clocks, you can look for tiny signals that are signatures of interesting new phenomena in physics.” 

“We're also constantly improving our time standards to support the measurement of evolving technologies in industry and science,” he added. 

While there have been other efforts around the world to replicate similar aspects of this project, this one will take place at one of the most elevated locations in the United States - an exciting feat for the research community. 

Mount Blue Sky, nestled in the Rocky Mountains of Colorado, is home to the highest paved road in North America peaking at 14,264 feet. This will allow the team to transport an optical atomic clock up the summit to measure geopotential differences corresponding to one centimeter changes in elevation. 

If successful, these measurements could open up new realms of how we use quantum and atomic physics for areas in hydrology, seismology, coastal mapping and geodetic surveying.  

The research team will first test these clocks at lower elevations before taking them ultimately up 14,000 feet in summer 2025. 

We sat down with Professor Scott Diddams for a deep dive into the ambitious project.  

What does your project entail with this new NSF grant?

Our project is really focused on using the best optical clocks — the most precise measurement tools ever made — to measure gravity. We think of the Earth as being just a sphere, but there’s actually significant variation in the Earth’s shape on large and small scales. Our plan is to use our clocks to measure those gravitational changes very precisely due to those features at different elevations. 

How will you achieve this? 

We're going to take one clock to the top of Mount Blue Sky and compare it to a local atomic clock in Boulder, Colo. This will be done via a laser link that transmits the clock’s rate over a laser beam through the air from Mount Blue Sky down to the Denver metro area. A challenge is that you don’t have a clear light of sight to Boulder, so we’ll have to go to a location — about 10 miles away — near the Broomfield area for that. We’ll use an optical fiber to connect from that location back to the reference clock at NIST.  

Atomic clock infographic
This perspective provides how the mobile optical clock atop Mt. Blue Sky will communicate with the transfer node in Broomfield, Colo. and connect with a reference optical clock in Boulder. 

What do you hope these atomic optical clocks will prove?  

When we compare their rates with the two clocks, we should see the one on the top of Mount Blue Sky ticking faster. By measuring the difference in the tick rates, we hope to make the most accurate test Einstein's predictions of general relativity. 

How can we relate this to everyday life? 

One thing that absolutely knows gravity is water, and water will flow to the lowest gravitational potential. And so in large coastal areas, determining elevation and the flow of water at the centimeter or a few centimeters level is quite important, particularly with climate change and rising sea levels. So our project will build a connection from very fundamental quantum science to a whole new area in geodesy and surveying as we know it. This is not a topic that you would initially think is connected to quantum physics. 

What makes Colorado’s Rocky Mountains uniquely fitting for this project?

We have this tremendous difference in elevation — or “relief,” in topographic terms — over a relatively short distance. There is around 9,000 feet of relief from Denver to Mount Blue Sky over a span of less than 50 miles, which we can use as leverage in the relative precision of our measurement. If we can measure the effect of that difference at the centimeter level, we stand to make the most precise measurement of the gravitational redshift. So that's pretty unique to Colorado. 

What excites you about the collaboration with NIST & NOAA?

This is a very unique team, and even more so that we are all here in Boulder. We have world experts in all the areas that are needed to make the project successful like being able to develop portable atomic optical clocks (Andrew Ludlow) and synchronize these clocks from the top of mountains down to the city (Laura Sinclair). We’ll also have a leading expert in geodesy (Derek van Westrum) who has actually already surveyed benchmarks in our labs with millimeter-level precision. 

Would you say this will be the highest altitude experiment you’ve ever conducted so far?

This probably will be the highest altitude experiment of its kind in the world. I have done short-term experiments with frequency combs on Mauna Kea in Hawaii, but that's 13,800 feet above sea level. I've never had an experiment at 14,000 feet yet, which makes this pretty unique. We're going to have to learn to efficiently work and operate the clock over extended periods in that high-altitude environment, as well.

Atomic Optical Clock Image Credit: Jesse Petersen