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arXiv:2608.07456 · Universe

Optimal Calibration-Free Observable for the Nucleon-Coupling Ratio in a Dual-Alkali Comagnetometer for Dark Matter Searches

Y. Rosenzweig, Yevgeny Kats, E. Sarid, Menachem Givon, Yonathan Japha, R. Folman

Published Oct 7, 2026• 0 citations
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Abstract

A dual-alkali single-cell 87Rb–39K–3He comagnetometer can read an axion-like dark matter signal through two optical rotation channels, encoding the ratio R=ξn/ξp of the field’s neutron and proton spin couplings in their relative response. The inter-species phase difference Δφ has been proposed as a calibration-free readout that is sensitive to R. Treating the extraction of R as a statistical estimation problem, we show that the estimate of R depends on the two channels only through their complex ratio, which splits into Δφ and an amplitude ratio, of which only Δφ is insensitive to the relative gain and hence calibration free. For our choice of comagnetometer parameters, above ∼100 Hz the phase difference alone captures most of the coupling-ratio information. At lower frequencies Δφ is no longer near-sufficient. Below ∼40 Hz, the amplitude ratio would improve the precision on R by a factor of ≳2. Recovering that information, however, requires the relative gain to be known sufficiently accurately, so Δφ remains the robust observable even where it is not the most informative one.

DOI
10.3390/universe12100298

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Magnetometers are among the most widely used probes in science and technology. Comagnetometers increase sensitivity by self-cancellation of magnetic noise, but only at low frequencies. We suggest a correlated measurement of two alkali species in one cell to cancel the magnetic background also at high frequencies. The inter-species phase difference of the light-matter interaction response function is found to be calibration free and insensitive to common-mode intensity noise. Utilizing a dark-matter signal as a testcase, the method achieves a thirtyfold background suppression, raising the signal-to-noise ratio by an order of magnitude or more, depending on the coupling to the different subatomic particles. We show that the method also provides model differentiation. The higher sensitivity and model differentiation open a path to novel probes for precision measurements in general and exotic fields in particular.
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Searching for physics beyond the Standard Model is one of the main tasks of experimental physics. Candidates for dark matter include axionlike ultralight bosonic particles. Comagnetometers form ultrahigh sensitivity probes for such particles and any exotic field that interacts with the spin of an atom. Here, we propose a multiatom-species probe that enables one not only to discover such fields and measure their spectrum but also to determine the ratios of their coupling strengths to the subatomic particles—electrons, neutrons, and protons. We further show that the multifaceted capabilities of this probe can be demonstrated with synthetic exotic fields generated by a combination of regular magnetic fields and light-induced fictitious magnetic fields in alkali atoms. These synthetic fields also enable the accurate calibration of any magnetometer or comagnetometer probe for exotic physics. Published by the American Physical Society 2024