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Scott Diddams
Professor and Lead Researcher

91��ý faculty and alumni are pushing quantum science to new heights — literally��

A team of scientists is attempting something no one has done before: Measuring Earth’s gravity at 14,000 feet using one of the most accurate clocks ever built.

Optical atomic clocks are instruments so precise they can detect tiny differences in the flow of time caused by Earth’s gravity.

“One of the most exciting things about quantum right now is that we’re finally moving from lab experiments to real-world environments,” said Scott Diddams, professor of electrical, computer and energy engineering and a lead researcher on the project. “We’re taking the best clock devices and testing them in entirely new ways.”

A portable optical atomic clock was transported this summer to the peak of Mt. Blue Sky — one of Colorado’s famed Fourteeners. Diddams and his colleagues then started to compare the time to another clock 9,000 feet below in Boulder through a one-of-a-kind free-space and fiber optic laser link.

Their ultimate goal: Make the most precise determination of whether a clock at higher elevations ticks at a faster rate than ones closer to Earth. If so, comparing how time flows between two elevations can unlock how we better understand our planet.

Albert Einstein’s theory of general relativity tells us that time will pass more slowly under the influence of gravity, known as the gravitational redshift.

The Mt. Blue Sky collaboration includes Diddams, NIST physicist Andrew Ludlow (PhDPhys’08); NIST physicist Laura Sinclair (PhDPhys’11), who enabled the frequency comb time transfer; and NOAA geodesist Derek van Westrum (PhDPhys’98), who provided millimeter-level benchmark measurements.

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What makes this endeavor remarkable is that we’re seeing how atoms inside an optical clock can reveal the gravitational pull on Earth — bridging the micro and the macro worlds.

“There’s not a lot of precedent for making measurements at this level,” said Ludlow, who developed the portable optical clock. “Then we’re adding into the mix that one of the clocks has to be up on a mountain top exposed to some harsh conditions.”

These optical clocks can measure changes in the Earth’s gravity down to just one centimeter in elevation, important in telling us where water flows, how land shifts and how the Earth responds to natural forces. Right now, that one-centimeter precision corresponds to measuring time at the 18th decimal point.

Although the team’s first trip to the Fourteener mainly tested whether the technology could survive the harsh mountain conditions — and it did — the researchers also successfully linked the Mt. Blue Sky clock to its twin in Boulder.

Next year, they will continue capturing detailed time comparisons at these extreme elevations, hoping to operate at the same precision as in the lab.

“This is about understanding the Earth,” Diddams said. “If we’re able to tell where water flows or what’s going on under the surface when we can’t directly see it, that’s something quite exciting. This can ultimately impact lives and property.”

While it may be obvious in Colorado that Mt. Blue Sky stands higher than Denver, Diddams noted that the position of the highest ground is much more challenging to determine precisely in flat coastal regions over long distances. A shift of even a few centimeters in elevation can determine whether floodwaters move toward communities or away from one.

“These clock-based tools can open up how we use powerful quantum-based measurements,” Diddams said. “We don’t know exactly what that’s going to yield, but we think it will give us new ways to measure the shape of the Earth.”