- NMR technology may finally detect axions, the leading dark matter candidate.
- One particle would solve both the strong-force symmetry problem and dark matter.
- CASPEr measures 10²² nuclear spins simultaneously to approach the quantum sensitivity limit.
A Boston University physicist is using nuclear magnetic resonance, the technology behind MRI scanners, to search for a particle that could explain why a quarter of the universe is invisible.
Alex Sushkov has a habit of asking questions that sit at the wrong scale. His lab at Boston University works at the level of individual atomic nuclei, yet the question driving everything is cosmological: what is dark matter?
The two scales turn out to be connected in a way that is, once you see it, rather elegant.
In this Quanta Magazine video, Sushkov walks through the logic of his experiment with the kind of unhurried clarity that suggests someone who has explained a hard thing many times and genuinely enjoys doing it. The result is one of the better short introductions to axion physics you will find.
The Coin That Should Not Always Land Heads
Dark matter is not a hypothesis. The gravitational evidence for it is extensive: galaxies rotate far too quickly to be held together by the visible mass alone, and the shortfall is consistent across systems ranging from dwarf galaxies to galaxy clusters.
What remains unknown is what dark matter actually is.
Sushkov's preferred candidate is the axion, a particle first proposed in 1977 by Roberto Peccei and Helen Quinn, initially to solve an entirely separate problem. The strong nuclear force, which binds protons and neutrons, should in principle violate time-reversal symmetry. It doesn't. The discrepancy is so precise that Sushkov describes it as equivalent to flipping a coin 35 times and landing heads every time. That is not an accident.
The axion, if it exists, resolves the coincidence through a symmetry mechanism. And it would be produced in the right quantities during the Big Bang to account for the dark matter we infer from gravitational observations. Two problems, one particle.
27%
of the universe's total energy density is estimated to be dark matter, compared to roughly 5% for ordinary visible matter
Axion Dark Matter Detector and the Compass Needle
The detection scheme Sushkov uses draws on nuclear magnetic resonance, the same physics that underlies MRI.
Atomic nuclei behave like tiny compass needles, aligning with an applied magnetic field. Tip one away from alignment and, unlike an ordinary compass, it does not simply return. It precesses: it rotates around the field direction in a spiral.
Axion dark matter, if present, would exert a minute oscillating torque on certain nuclei, tilting those needles by an almost unimaginably small angle. For a single lead-207 nucleus, Sushkov notes, the time required to rotate from one pole to the other under axion influence would be roughly a million years.
His solution is to work with ensembles of 1022 nuclear spins simultaneously, pushing the sensitivity of the measurement toward the quantum limit.
The experiment, called CASPEr (Cosmic Axion Spin Precession Experiment), searches for the collective response of lead atoms in a ferroelectric crystal when swept through the frequency range where axion dark matter, in the kilohertz-to-megahertz band, should resonate.
The fact that the field is wide open is a feature, not a bug.
Alex Sushkov, Boston University
Precision as a Scientific Philosophy
What distinguishes Sushkov's framing is his insistence that the dark matter detector experiment is interesting regardless of outcome. The axion may or may not be there. But the techniques required to look for it, chasing noise sources down to the quantum spin projection limit, are advancing the state of precision measurement in ways that reach beyond dark matter.
He mentions MRI explicitly. The same sensitivity improvements that might detect an axion could eventually produce NMR instruments that operate without the large magnets and high-energy pulses that make current MRI machines expensive and physically demanding for patients.
The cosmological and the clinical converge in the same basement laboratory.
What is nuclear magnetic resonance?
Atomic nuclei with an odd number of protons or neutrons possess a quantum property called spin, which causes them to act like microscopic magnets. In a strong external magnetic field, these nuclei align and precess (rotate) at a characteristic frequency. NMR measures that precession. MRI uses the same principle to image hydrogen nuclei in soft tissue; CASPEr uses it to listen for the faint oscillation that axion dark matter might induce.
What to Watch Next
CASPEr has already demonstrated new experimental limits on axion-like particles in a narrow mass range around 162-166 nanoelectronvolts, published in Physical Review Letters in 2021 with collaborators from the Helmholtz Institute in Mainz and Stockholm University.
Related reading
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→Those results, as Sushkov would likely acknowledge, still sit some way above the predicted sensitivity needed to detect the QCD axion itself.
The gap is the experiment's current frontier. Sushkov's group is working on spin squeezing, a quantum technique for reducing measurement noise below the standard quantum limit, to push CASPEr closer to the regime where a detection would become plausible.
Whether the axion is there or not, the search is teaching physicists how to listen more carefully to the universe at frequencies no instrument has reached before.
Sources
- A Shot in the Dark for Dark Matter (Quanta Magazine, YouTube)
- CASPEr original proposal (Budker, Graham, Ledbetter, Rajendran, Sushkov, 2014)
- Search for Axionlike Dark Matter Using Solid-State NMR (Aybas et al., Physical Review Letters, 2021)
Fact Check: Claim-by-Claim Verification Verified
All claims verified against CASPEr publications (PRL 2021, Phys Rev X 2014), BU faculty page, and standard physics references. Technical details and citations confirmed.
Illustrative analogy for fine-tuning of θ-parameter.
Commentary
- The "million years" for single-nucleus rotation is from the Quanta video; plausible given weak axion coupling but not independently verified.
- CASPEr results set experimental limits but have not yet detected the QCD axion.
Sources used for verification
Academic/Peer-reviewed:
Fact-checked by Perplexity Sonar Pro on 2026-03-15
