NIST Physicist’s Once Obscure Work Is Now Helping Researchers Learn About the Origins of the Universe

When physicist John “Ben” Mates completed his doctoral thesis in 2011, he figured few people would read it. 

It’s not that Mates, who conducted his Ph.D. research at NIST while a graduate student at the University of Colorado, thought his work unimportant. 

Mates was just being realistic. Most scientists don’t bother to wade through doctoral dissertations, which can run more than 100 pages. Dissertations tend to focus on highly specialized topics. 

And for several years of his career at NIST, Mates was right.

He had devised a novel method to read out the signals from an array of exquisitely sensitive sensors that measure tiny changes in the intensity of thermal radiation (heat), including the afterglow of the Big Bang, known as the cosmic microwave background (CMB).

Reading out data from the detectors, developed at NIST and known as transition edge sensor (TES) bolometers, had proved challenging. That’s because the bolometers can only operate at temperatures a fraction of a degree above absolute zero, which is about minus 273 degrees Celsius or minus 459 degrees Fahrenheit. If too many wires link the ultracold detectors to room-temperature equipment, the sensors will heat up and stop functioning.

Mates’ dissertation described a way to minimize the number of these wires, enabling the sensors to maintain their chilly operating temperature. 

After completing his thesis, Mates pursued another research project at NIST. 

(Many) Bolometers Needed 

In late 2013, however, his NIST supervisor, Joel Ullom, asked him if he’d like to return to his original study. His dissertation, Ullom told Mates, had taken on added importance. 

Mates had previously demonstrated that signals from two of the TES bolometers could be read out using a single wire connected to a room-temperature device rather than using a separate wire for each sensor. 

Although he had designed the method to minimize the room-temperature connections for a much larger number of sensors, he had not actually shown it could work.

Now, that demonstration was urgently needed — and on a massive scale. 

Astronomers wanted to use not just two but thousands of the TES bolometers on a set of ground-based telescopes to examine the CMB with 10 times more sensitivity than ever before. Although researchers have studied the CMB for decades, the bolometers are able to capture details of the tiny temperature variations in the radiation that may put to the test the leading theory of how the universe was born.

With thousands of bolometers, however, it would be virtually impossible to attach a separate room-temperature wire to each one without heating the sensors beyond their operating temperature.

Over the next 10 years, Mates perfected his technique, showing how the signal from each TES — a change in a tiny current — could be converted to a unique frequency. Thousands of those frequencies, he showed, could be carried on a single room-temperature cable, dramatically reducing the flow of heat back to the detectors.

Using his method, known as microwave multiplexing, astronomers recently installed 67,080 bolometers on the Simons Observatory, a suite of four telescopes in Chile devoted to studying the CMB. 

The NIST-designed sensors act like miniature thermometers and can discern tiny temperature variations — as small as ten-millionths of a degree — in the CMB over more than 40% of the sky. 

The minuscule hot and cold spots correspond to slight variations in the density of the universe in its infancy, 380,000 years after its violent birth. Studying those variations reveals how and where tiny clumps of matter, the seeds of the galaxies we see in the sky today, first formed in the cosmos.

The bolometers also record patterns of different polarizations in the CMB — wiggles in the electric field of the radiation. Those wiggles encode a wealth of information about the universe an instant after the Big Bang and could hold clues about its mysterious beginning.

Multiplexing Research Goes Mainstream 

Now Mates’ dissertation is a hot topic — required reading for many scientists interested in multiplexing. He’s gotten hundreds of requests for reprints and has traveled around the world, recently installing instrumentation at the Japan Proton Accelerator Research Complex in Tokai. 

“It’s sort of freakish how it all worked out,” Mates said. “I never imagined the work would have such an impact.”

His thesis is so popular that Mates said he’s considering publishing an updated version of his manuscript.

In the future, Mates hopes to keep refining the technique and reducing the cost, so there can be many more projects over the next decade or longer. 

While he appreciates the attention his work is currently receiving, for Mates, the measurement problems were motivation to keep researching. 

“I think I also find most of the problems of developing and improving the system to be interesting on their own,” he said.