Searching For Dark Matter With Quantum Computers, One Blip Of Light At A Time

Searching For Dark Matter With Quantum Computers, One Blip Of Light At A Time
Graphic: Jim Cooke (Gizmodo), Photo: Getty

For decades, scientists have struggled to define the stuff that comprises a quarter of the universe: dark matter. As experiments continue to turn up empty-handed, one team hopes to find dark matter by incorporating the laws that govern subatomic particles”quantum mechanics”with a nascent kind of technology called a quantum computer. It’s what’s brought Fermilab scientist Daniel Bowring, whose background is in accelerator physics, into the quantum computing laboratory of collaborator David Schuster at the University of Chicago. “The first time I went in there, I felt like Charlie entering Willie Wonka’s factory,” he said.

Today, the experiment lab is at Fermilab, just outside Batavia, Illinois, in a high-ceilinged room with all-white walls and a dark tower of physics equipment at the base of a staircase in a warehouse, sectioned off by glass dividers. When I visited Fermilab in January, typical quantum computing components sat in the far wall: a tower of electronics with flashing lights and a table with a computer monitor beside a person-sized silver cylinder hanging from a steel rack, called a dilution refrigerator, which keeps superconducting components functioning at just above absolute zero. The room echoed with a rhythmic squeal as the refrigerator pumped its liquid helium, while Bowring, Fermilab research associate Rakshya Khatiwada, and University of Chicago graduate students Akash Dixit and Ankur Agrawal showed me how it worked. It’s called QISMET, short for Quantum Information Science Metrology, though Bowring hates acronyms.

Today’s quantum computers are limited in their abilities, excelling only in a few contrived algorithms useful mainly as random number generators, though quantum computer makers hope these devices will one day solve problems that regular computers can’t. But sensor limits have driven one dark matter-hunting team to build a dark matter detector from the same guts as a quantum computer. Their device under construction at Fermilab solidifies extreme sensing as one of present-day quantum technology’s best real-world applications.

Bowring realises that the experiment combines two over-hyped physics buzzwords, and he worries about how hype in the field might affect funding for quantum science more generally. “When I tell people I’m using qubits to look for dark matter,” he said, “I feel a little bit silly and usually try hard to explain that we came about it honestly, and we genuinely think this technology is the most compelling technology to allow us to look for higher-mass axions.”

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Axions are popular because, like WIMPs, they’d solve both the mystery of dark matter in space as well as a mystery surrounding the behaviour of subatomic particles. In the axion’s case, that problem is called the strong-CP problem. The force that holds atomic nuclei together is called the strong force, and, based on what we know about other forces, there’s no reason for the laws of physics to be the same if you swap a particle with an identical particle but with the opposite charge (C) and parity (P). Yet somehow, particles experiencing the strong force maintain this symmetry. Physicists in the 1960s, Roberto Peccei and Helen Quinn, devised a theory to help explain the apparent conservation of these properties. Later, physicists Frank Wilczek and Steven Weinberg realised that the theory made room for an extremely light new particle called the axion. But if axions existed, they would have properties amenable to explaining “cold” dark matter, the kind of dark matter that cosmologists think fills the universe: abundant, slow-moving particles that only experience the gravitational force over long distances. So physicists have set about searching for signs of them.

Device measuring qubits at Fermilab in Illinois. (Photo: Ryan Mandelbaum, Gizmodo)

Bowring comes from the Axion Dark Matter eXperiment, or ADMX, among the most famous axion-hunting experiments. ADMX is an antennae placed in a magnetic field in an underground cavity, with a rotating tuning rod that adjusts the frequency of microwave photons”particles of electromagnetic radiation”to which it’s sensitive. Physicists theorise that axions turn into photons in the presence of a strong magnetic field, and the ADMX experiment involves slowly rotating the tuning rod to sweep through certain frequencies of photons, like tuning a radio dial. Except the signal is so weak that it’s like trying to pick up a phone call on Mars using a cell tower on Earth, Bowring said.

If the axion-originating photons have frequencies higher than a few GHz, then the photons emitted by any object in the experiment with a temperature can drown out the signal, even with the help of components like amplifiers. Amplifiers come with their own limits; those used in ADMX require knowledge of the photon’s phase and amplitude at the same time. But a core theory of quantum mechanics, the uncertainty principle, says that certain combinations of properties cannot be simultaneously measured precisely, combinations including amplitude and phase. Researchers just want to know whether axions are there or not, so they needed a system that could maximise the experiment’s sensitivity to a photon’s amplitude, regardless of what its other properties are.

The ADMX team members realised that they could use quantum computing expertise to solve their quantum problem. “It’s not enough to care about microwave photons,” Bowring explained. “We needed people who care about one microwave photon at a time.” That brought them to the quantum computing community.

A qubit at Fermilab in Illinois. (Photo: Ryan Mandelbaum, Gizmodo)

In 2007, physicist David Schuster, then at Yale, asked his advisor whether qubits could serve as useful detectors for astronomy”after all, the core component of the quantum computer, the qubit, is essentially just a super-sensitive light detector. But his advisor responded that the device wouldn’t work for astronomy, since it wouldn’t detect photons unless they magically appeared inside of the cavity housing the qubit. “It was sensitive, but only good at detecting things in the cavity,” Schuter said.

Nearly a decade later, Schuster, now at professor at the University of Chicago, was visiting Fermilab to talk about superconducting radiofrequency cavities with another team and bumped into Fermilab and ADMX physicist Aaron Chou. Chou had heard about Schuster’s qubits and knew about Schuster’s interest in using qubits for sensing. Theory says that when axions interact inside of a very strong magnetic field, they turn into photons”causing them to magically appear inside of a box. They’d found an application for which quantum computing could lead to a useful astronomy experiment.

Quantum computers in the technical sense are only computers when you use them to perform computations; otherwise, their qubits are just systems of artificial atoms. The most popular qubit architecture consists of loops of superconducting wire through which current travels without resistance, broken up by a small piece of insulator in a region called a Josephson junction. Each of these loops obey the same rules of quantum mechanics that an electron would in its orbit around an atom: in the presence of a photon of the right frequency, they enter an excited state, represented in the loop as a tiny amount of current through the wire. But unlike an atom’s electron, qubits light up in response to a range of photon frequencies rather than individual frequencies like an atom, Bowring explained.

QISMET isn’t quite a quantum computer in that it doesn’t do any computations, but it’s based on the same technology. Each of the QISMET superconducting qubits is a strip of glass that can sit on your fingertip with a pair of antennae etched in, black lines in the glass visible to the naked eye if you squint. The superconducting loop is microscopic, invisible between the black lines.

Superconducting radiofrequency cavity hooked up to a diluting fridge at Fermilab in Illinois”part of the Dark SRF experiment run by physicist Alex Romanenko.

Dixit, one of the University of Chicago graduate students, walked me through the axion detection process: First, place a radiofrequency cavity, an empty box whose walls act like mirrors to trap photons inside, inside of a strong magnet, so the axions turn into photons. Should a photon appear, move that photon into another radiofrequency cavity containing a qubit (superconducting technology can struggle in a strong magnetic field). Use radio pulses to measure the qubit over and over again, seeing if it’s in an excited state more often than randomness alone would allow, and only when the magnet’s on. If yes, then (barring any other interpretation) QISMET detected axions.

The small team went from an empty lab to a proof-of-concept system in less than a year, Khatiwada told me. She’d joined QISMET as a cryogenic electronics expert from ADMX and was drawn to the qubit-based photon sensor experiment. “I want to do an experiment that’s preferably the most sensitive experiment looking for axions,” she said. “It was just this need to make the search better and more sensitive really.”

QISMET suffers from the same issue that other quantum computers do, explained Dixit. “We know qubits can count photons, but they also make a lot of mistakes,” he said. “We want to know how to take all of these mistakes into account.” That means ensuring that the cavity is as empty as possible and stores the axion’s photon for as long as possible and that researchers understand the potential for the qubit to accidentally flip to the excited state unprovoked. Chou said in an email that it could be another year before the team finishes ironing out the experiment’s kinks.

Other scientists have started to incorporate quantum intuition into dark matter searches of their own, such as the axion-hunting HAYSTAC experiment or Fermilab scientist Alex Romanenko’s Dark SRF experiment, which attempts to produce a dark matter candidate in a superconducting radiofrequency cavity and detect it in another.

Pursuing these experiments has pushed these two fields forward hand-in-hand, Fermilab deputy chief technology officer Anna Grasselino told Gizmodo. “I would say the technology is pushing forward the search, but the search itself is giving us motivation to further explore technology in the quantum regime,” she said. Quantum technologies for computing and for axion hunting share a related but ultimately different goal, which pushes the field forward overall; Dixit said that most companies working on building quantum computers don’t think about qubit errors at quite the same level that the QISMET team must”they require some of the lowest error rates.

Among the biggest challenges in running a fundamental science experiment like this, said Bowring, is workforce development. IBM, Google, Intel, Microsoft, and other big-money companies are all pursuing quantum technology in a space with a relatively small pool of potential talent. Bowring can offer a candidate about to finish grad school a post-doctoral researcher’s salary, while a tech company can offer several times that. “We can only go as fast as we have staff power for,” he said.

But once QISMET is up and running, it will demonstrate a real benefit of quantum technology over existing sensing solutions, likely before companies like Google and IBM’s quantum computers have useful computing applications. The work demonstrates the importance of basic research in pushing the boundaries of technology to solve problems only physicists have, like how to find a subatomic particle that might not exist. The experiment’s scientists don’t endeavour to develop a product that might one day generate a profit; they’re driven by the powerful, mysterious force of curiosity.