Current models suggest that dark matter makes up over a quarter of all the matter and energy in our universe and is about five times more abundant than ordinary matter. However, as dark matter is invisible—emitting no light or energy—it cannot be detected using conventional sensors and detectors.

Now, researchers from the Accelerator Technology & Applied Physics (ATAP) Division at Berkeley Lab, in collaboration with colleagues from Princeton University and the California Institute of Technology, have proposed a novel approach that uses quantum sensing of the spin of helium-3 (³He) atoms—a stable (and the least abundant) isotope of helium—evaporated from the surface of a film of liquid helium to detect dark matter particles. The work, part of a multi-divisional project led by the Lab’s Physics Division, promises a new technique that extends the search for dark matter into mass ranges below the detection threshold of current detectors and could lead to more powerful sensors for applications in particle physics, astrophysics, and cosmology.

“When we look at the mass balance of the universe, only a small percentage, around five percent, is made up of matter we can see,” explains Thomas Schenkel, a senior scientist and head of ATAP’s Fusion Science & Ion Beam Technology Program, who initiated the collaboration.

“The vast majority of matter is dark matter, named because we don’t know what properties to assign to it. However, we know dark matter exists because it interacts with normal matter through gravity.”

Because dark matter constitutes the vast majority of the universe’s mass, it has guided the evolution and structure of the universe and the formation of visible, everyday matter. Searching for dark matter, Schenkel says, is therefore “essential to understanding the universe’s size, shape, and future.”

The search for dark matter

The search for dark matter has been highlighted as a top priority in the recently released 2023 P5 Report: A Roadmap for Particle Physics. The report, which outlines particle physicists’ recommendations for the next decade, states that: “Answering some of the deepest questions about particle physics itself requires … revealing the underlying nature of dark matter …” and that: “Developments in detector instrumentation lay the foundation for future campaigns to identify dark matter in new scenarios.”

“This search,” notes Schenkel, “has been a decades-long effort by scientists from around the world, with the direct detection of dark matter primarily focused on so-called weakly interacting massive particles, or WIMPs, which scatter dark matter candidates via nuclear recoils and release energy in the GeV range.”

However, as the mass of dark matter candidates drops below around 1 GeV (about the mass of a proton), the resulting scattering energy can fall below the detection threshold or get buried in detector noise.

“Searches for WIMPs have achieved amazing sensitivity but, to date, have not confirmed the existence of high-mass dark matter particles,” says Schenkel. “Consequently, scientists are focusing on searching for dark matter in energy ranges well below 1 GeV, including the Helium Roton Apparatus for Light Dark Matter (HeRALD) program led by Dan McKinsey of the Lab’s Physics Division, which aims to detect low-mass dark matter candidates and in which the proposed technique could be used. This has become a hot topic for seeking novel approaches that probe for dark matter candidates as light as a few keV.”

In recent years, progress in dark matter theory has spurred advances in direct-detection technologies with quantum sensing. This emerging sensor technology detects events by tracking quantum properties (such as spin coherence) in a detector or sensor.

Schematic of the dark matter (DM) detector concept. An interaction with DM in an ionic crystal generates ∼1 meV phonons, which impinge on a surface covered with a van der Waals helium film. The phonon quantum evaporates a ³He atom from the film’s surface, which is then collected on the van der Waals film covering the detector structures. The ³He atoms diffuse until captured by an electron bound to the helium surface in a charge-coupled device (CCD)-like structure. Periodically, the collected ³He atoms are moved with the CCD to a readout device, which operates via nuclear spin-induced decoherence of an electron in a spin-based quantum sensor.

In light of these advances, the researchers have proposed a detector system based on quantum sensing that uses low-energy phonons—quanta of vibrational energy that are analogous to photons of light—created through rare interactions of dark matter with an absorber, such as a polar crystal like sodium iodide.

“This absorber is coated with a thin helium film that interacts with potential dark matter particles, which leads to the quantum evaporation of helium-3 atoms,” explains Schenkel. These atoms are then collected on an adjacent surface and transported to a readout system with a charged coupling device patterned onto the collector surface.

He says the readout system is at the heart of the quantum sensor and operates by tracking the spin decoherence of the spin of electrons induced by the nuclear spins of the helium-3 atoms. He notes that detecting single nuclear spins has already been demonstrated in several ways, including using electrons in quantum dots or color centers.

Schenkel says the basic principle behind the detector is to use quantum coherence to track the rare and small energy transfer events when dark matter particles in the lower-mass regime interact with liquid helium films at greater sensitivity.

This approach, he says, combines elements from previous work on spin coherence with results from studies of electrons on liquid helium, pioneered by Stephen Lyon from Princeton, and theoretical studies by Kathryn Zurek of the California Institute of Technology, which addressed the problem of detecting these infrequent, very low energy transfers.

“The technique would allow us to search regions of the dark matter mass maps not yet explored and could also be tuned for different binding energies of the helium-3 atoms, allowing us to change the threshold for the region of dark matter to be explored,” says Schenkel. “Quantum sensing could become part of future upgrades to dark matter projects.”

The work was supported by the Department of Energy Office of High Energy Physics for the Quantum Information Science Enabled Discovery (QuantISED) program. Maurice Garcia-Sciveres, a senior scientist in the Physics Division at Berkeley Lab, leads QuantISED.

To learn more …

1.  A. Lyon, Kyle Castoria, Ethan Kleinbaum, Zhihao Qin, Arun Persaud, Thomas Schenkel, and Kathryn M. Zurek. “Single phonon detection for dark matter via quantum evaporation and sensing of ³He,” Phys. Rev. D109, 023010, 2024, https://doi.org/10.1103/PhysRevD.109.023010
2. More information on HeRALD: S. A. Hertel, A. Biekert, J. Lin, V. Velan, and D. N. McKinsey. “Direct detection of sub-GeV dark matter using a superfluid 4He target”, Rev. D. 100, 092007, 2019, https://doi.org/10.1103/PhysRevD.100.092007, and https://tesseract.lbl.gov/herald
3. Newly Released Roadmap for Particle Physics Includes Support for Berkeley Lab Projects

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