Looking for dark matter differently – Physics World

Looking for dark matter differently – Physics World

Time Stamp: April 22, 2024 4:00 AM


looking for frequent interactions between nuclei in a detector and low-energy dark matter
The proposed new dark matter detection method would look for frequent interactions between nuclei in a detector and low-energy dark matter that may be present in and around Earth. (Right) A conventional direct detection experiment looks for occasional recoils from dark matter scattering. Courtesy: Anirban Das, Noah Kurinsky and Rebecca Leane

Dark matter makes up about 85 percent of the universe’s total matter, and cosmologists believe it played a major role in the formation of galaxies. We know the location of this so-called galactic dark matter thanks to astronomical surveys that map how light from distant galaxies bends as it travels towards us. But so far, efforts to detect dark matter trapped within the Earth’s gravitational field have come up empty-handed, even though this type of dark matter – known as thermalized dark matter – should be present in greater quantities.

The problem is that thermalized dark matter travels much more slowly than galactic dark matter, meaning its energy may be too low for conventional instruments to detect. Physicists at the SLAC National Laboratory in the US have now proposed an alternative that involves searching for thermalized dark matter in an entirely new way, using quantum sensors made from superconducting quantum bits (qubits).

An entirely new approach

The idea for the new method came from SLAC’s Noah Kurinsky, who was working on re-designing transmon qubits as active sensors for photons and phonons. Transmon qubits needs to be cooled to temperatures near absolute zero (- 273 °C) before they become stable enough to store information, but even at these extremely low temperatures, energy often re-enters the system and disrupts the qubits’ quantum states. The unwanted energy is typically blamed on imperfect cooling apparatus or some source of heat in the environment, but it occurred to Kurinsky that it could have a much more interesting origin: “What if we actually have a perfectly cold system, and the reason we can’t cool it down effectively is because it’s constantly being bombarded by dark matter?”

While Kurinsky was pondering this novel possibility, his SLAC colleague Rebecca Leane was developing a new framework for calculating the expected density of dark matter inside Earth. According to these new calculations, which Leane performed with Anirban Das (now a postdoctoral researcher at Seoul National University, Korea), this local dark-matter density could be extremely high at the Earth’s surface – much higher than previously thought.

“Das and I had been discussing what possible low threshold devices could probe this high predicted dark matter density, but with little previous experience in this area, we turned to Kurinsky for vital input,” Leane explains. “Das then performed scattering calculations using new tools that allow the dark matter scattering rate to be calculated using the phonon (lattice vibration) structure of a given material.”

Low energy threshold

The researchers calculated that a quantum dark-matter sensor would activate at extremely low energies of just one thousandth of an electronvolt (1 meV). This threshold is much lower than that of any comparable dark matter detector, and it implies that a quantum dark-matter sensor could detect low-energy galactic dark matter as well as thermalized dark matter particles trapped around the Earth.

The researchers acknowledge that much work remains before such a detector ever sees the light of day. For one, they will have to identify the best material for making it. “We were looking at aluminium to start with, and that’s just because that’s probably the best characterized material that’s been used for detectors so far,” Leane says. “But it could turn out that for the sort of mass range we’re looking at, and the sort of detector we want to use, maybe there’s a better material.”

The researchers now aim to extend their results to a broader class of dark matter models. “On the experimental side, Kurinsky’s lab is testing the first round of purpose-built sensors that aim to build better models of quasiparticle generation, recombination and detection and study the thermalization dynamics of quasiparticles in qubits, something that is little understood,” Leane tells Physics World. “Quasiparticles in a superconductor seem to cool much less efficiently than previously thought, but as these dynamics are calibrated and modelled better, the results will become less uncertain and we may understand how to make more sensitive devices.”

The study is detailed in Physical Review Letters.

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