Thanks to low-noise superconducting quantum amplifiers invented at UC Berkeley, physicists are now embarking on the most sensitive search yet for axions, one of today’s top candidates for dark matter.

The Axion Dark Matter Experiment (ADMX) reported results today showing that it is the world’s first and only experiment to have achieved the necessary sensitivity to “hear” the telltale signs of dark matter axions.

The milestone is the result of more than 30 years of research and development, with the latest piece of the puzzle coming in the form of a quantum device that allows ADMX to listen for axions more closely than any experiment ever built.

John Clarke, a professor of physics in the graduate school at UC Berkeley and a pioneer in the development of sensitive magnetic detectors called SQUIDs (superconducting quantum interference devices), developed the amplifier two decades ago. ADMX scientists, with Clarke’s input, have now incorporated it into the ADMX detector at the University of Washington, Seattle, and are ready to roll.

“ADMX is a complicated and quite expensive piece of machinery, so it took a while to build a suitable detector so that they could put the SQUID amplifier on it and demonstrate that it worked as advertised. Which it did,” Clarke said.

The ADMX team published their results online today in the journal Physical Review Letters.

“This result signals the start of the true hunt for axions,” said Andrew Sonnenschein at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, the operations manager for ADMX. “If dark matter axions exist within the frequency band we will be probing for the next few years, then it’s only a matter of time before we find them.”

A cutaway rendering of the ADMX detector, which can detect axions producing photons within its cold, dark interior. ADMX collaboration

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Dark matter: MACHOs, WIMPs or axions?

Dark matter is the missing 84 percent of matter in the universe, and physicists have looked extensively for many possible candidates, most prominently massive compact halo objects, or MACHOs, and weakly interacting massive particles, or WIMPs. Despite decades of searching for MACHOs and WIMPs, scientists have struck out; they can see the effects of dark matter in the universe, in how galaxies and stars within galaxies move, but they can’t see dark matter itself.

Axions are becoming the favored alternative, in part because their existence would also solve problems with the standard model of particle physics today, including the fact that the neutron should have an electric dipole moment, but doesn’t.

Like other dark-matter candidates, axions are everywhere but difficult to detect. Because they interact with ordinary matter so rarely, they stream through space, even passing through the Earth, without “touching” ordinary matter. ADMX employs a strong magnetic field and a tuned, reflective box to encourage axions to convert to microwave-frequency photons, and uses the quantum amplifier to “listen” for them. All this is done at the lowest possible temperature to reduce background noise.

Clarke learned of a key stumbling block for ADMX in 1994, when meeting with physicist Leslie Rosenberg, now a professor at the University of Washington and chief scientist for ADMX, and Karl van Bibber, now chair of UC Berkeley’s Department of Nuclear Engineering. Because the axion signal would be very faint, any detector would have to be very cold and “quiet.” Noise from heat, or thermal radiation, is easy to eliminate by cooling the detector down to 0.1 Kelvin, or roughly 460 degrees below zero Fahrenheit. But eliminating the noise from standard semiconductor transistor amplifiers proved difficult.

They asked Clarke, would SQUID amplifiers solve this problem?

Supercold amplifiers lower noise to absolute limit

Though he had built SQUID amplifiers that worked up to 100 MHz frequencies, none worked at the gigahertz frequencies needed, so he set to work to build one. By 1998, he and his collaborators had solved the problem, thanks in large part to initial funding from the National Science Foundation and subsequent funding from the Department of Energy (DOE) through Lawrence Berkeley National Laboratory. The amplifiers on ADMX were funded by DOE through the University of Washington.

Clarke and his group showed that, cooled to temperatures of tens of milliKelvin above absolute zero, the Microstrip SQUID Amplifier (MSA) could achieve a noise that was quantum limited, that is, limited only by Heisenberg’s Uncertainty Principle.

“You can’t do better than that,” Clarke said.

This much quieter technology, combined with the refrigeration unit, reduced the noise by a factor of about 30 at 600 MHz so that a signal from the axion, if there is one, should come through loud and clear. The MSA currently in operation on ADMX was fabricated by Gene Hilton at the National Institute of Standards and Technology in Boulder, Colorado, and tested, calibrated and packaged by Sean O’Kelley, a graduate student in Clarke’s research group at UC Berkeley.

The ADMX team plans to slowly tune through millions of frequencies in hopes of hearing a clear tone from photons produced by axion decay.

“This result plants a flag,” said Rosenberg. “It tells the world that we have the sensitivity, and have a very good shot at finding the axion. No new technology is needed. We don’t need a miracle anymore, we just need the time.”

Clarke noted too that the high-frequency, low-noise quantum SQUID amplifiers he invented for ADMX have since been employed in another hot area of physics, to read out the superconducting quantum bits, or qubits, for quantum computers of the future.

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Forty years ago, scientists theorized a new kind of low-mass particle that could solve one of the enduring mysteries of nature: what dark matter is made of. Now a new chapter in the search for that particle has begun.

This week, the Axion Dark Matter Experiment (ADMX) unveiled a new result, published in the journal Physical Review Letters, that places it in a category of one: it is the world’s first and only experiment to have achieved the necessary sensitivity to “hear” the telltale signs of dark matter axions. This technological breakthrough is the result of more than 30 years of research and development, with the latest piece of the puzzle coming in the form of a quantum-enabled device that allows ADMX to listen for axions more closely than any experiment ever built.

 

Inside the ADMX experiment hall at the University of Washington. The ADMX detector is underground, surrounded by a magnetic field.Mark Stone/University of Washington

ADMX is based at the University of Washington and managed by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. This new result, the first from the second-generation run of ADMX, sets limits on a small range of frequencies where axions may be hiding, and sets the stage for a wider search in the coming years.

“This result signals the start of the true hunt for axions,” said Fermilab’s Andrew Sonnenschein, the operations manager for ADMX. “If dark matter axions exist within the frequency band we will be probing for the next few years, then it’s only a matter of time before we find them.”

One theory suggests that the dark matter that holds galaxies together might be made up of a vast number of low-mass particles, which are almost invisible to detection as they stream through the cosmos. Efforts in the 1980s to find this particle, named the axion by theorist Frank Wilczek, currently of the Massachusetts Institute of Technology, were unsuccessful, showing that their detection would be extremely challenging.

 

Gray Rybka (left), co-spokesperson for ADMX and assistant professor of physics at the University of Washington, and Leslie Rosenberg (right), professor of physics at the University of Washington and chief scientist for ADMX, at the ADMX experiment site on the University of Washington campus in Seattle.Mark Stone/University of Washington

ADMX is an axion haloscope — essentially a large, low-noise, radio receiver, which scientists tune to different frequencies and listen to find the axion signal frequency. Axions almost never interact with matter, but with the aid of a strong magnetic field and a cold, dark, properly tuned, reflective box, ADMX can “hear” photons created when axions convert into electromagnetic waves inside the detector.

 

View of the top portion of the ADMX detector at the University of Washington.Mark Stone/University of Washington

“If you think of an AM radio, it’s exactly like that,” said Gray Rybka, co-spokesperson for ADMX and assistant professor of physics at the University of Washington. “We’ve built a radio that looks for a radio station, but we don’t know its frequency. We turn the knob slowly while listening. Ideally we will hear a tone when the frequency is right.”

This detection method, which might make the “invisible axion” visible, was invented by Pierre Sikivie of the University of Florida in 1983, as was the notion that galactic halos could be made of axions. Pioneering experiments and analyses by a collaboration of Fermilab, the University of Rochester and the U.S. Department of Energy’s Brookhaven National Laboratory, as well as scientists at the University of Florida, demonstrated the practicality of the experiment. This led to the construction in the late 1990s of a large-scale detector at the U.S. Department of Energy’s Lawrence Livermore National Laboratory that is the basis of the current ADMX.

It was only recently, however, that the ADMX team has been able to deploy superconducting quantum amplifiers to their full potential enabling the experiment to reach unprecedented sensitivity. Previous runs of ADMX were stymied by background noise generated by thermal radiation and the machine’s own electronics.

 

A cutaway rendering of the ADMX detector, which can detect axions producing photons within its cold, dark interior.ADMX collaboration

Fixing thermal radiation noise is easy: a refrigeration system cools the detector down to 0.1 Kelvin (roughly -460 degrees Fahrenheit). But eliminating the noise from electronics proved more difficult. The first runs of ADMX used standard transistor amplifiers. Then, the researchers connected with John Clarke, a professor at the University of California Berkeley, who developed a quantum-limited amplifier for the experiment. This much quieter technology, combined with the refrigeration unit, reduces the noise by a significant enough level that the signal, should ADMX discover one, will come through loud and clear.

“The initial versions of this experiment, with transistor-based amplifiers would have taken hundreds of years to scan the most likely range of axion masses. With the new superconducting detectors we can search the same range on timescales of only a few years,” said Gianpaolo Carosi, co-spokesperson for ADMX and scientist at Lawrence Livermore National Laboratory.

“This result plants a flag,” said Leslie Rosenberg, professor of physics at the University of Washington and chief scientist for ADMX. “It tells the world that we have the sensitivity, and have a very good shot at finding the axion. No new technology is needed. We don’t need a miracle anymore, we just need the time.”

ADMX will now test millions of frequencies at this level of sensitivity. If axions are found, it would be a major discovery that could explain not only dark matter, but other lingering mysteries of the universe. If ADMX does not find axions, that may force theorists to devise new solutions to those riddles.

 

Gray Rybka (co-spokesperson for ADMX and assistant professor of physics at the University of Washington) at the top of the ADMX detector on the University of Washington campus in Seattle. The ADMX detector is underground, surrounded by a magnetic field.Mark Stone/University of Washington

“A discovery could come at any time over the next few years,” said scientist Aaron Chou of Fermilab. “It’s been a long road getting to this point, but we’re about to begin the most exciting time in this ongoing search for axions.”

The ADMX collaboration includes scientists at Fermilab, the University of Washington, Lawrence Livermore National Laboratory, Pacific Northwest National Laboratory, Los Alamos National Laboratory, the National Radio Astronomy Observatory, the University of California at Berkeley, the University of Chicago, the University of Florida and the University of Sheffield. This research is supported by the U.S. Department of Energy Office of Science, the Heising-Simons Foundation and research and development programs at the U.S. DOE’s Lawrence Livermore National Laboratory and the U.S. DOE’s Pacific Northwest National Laboratory.

Quelle: Fermilab and the University of Washington.