Search for Ultralight Dark Matter with Quantum Magnetometry in the Earth's Cavity

TL;DR

The work targets ultralight dark matter (ULDM) detection via the Earth's cavity using an unshielded quantum magnetometer. They develop a theoretical framework linking axion-photon coupling $g_{a\gamma\gamma}$ and dark photon kinetic mixing $\\epsilon$ to geomagnetic signals through an effective current expanded in vector spherical harmonics, predicting nearly monochromatic magnetic fields at the DM Compton frequency with a bandwidth set by the DM velocity distribution. In a one-hour desert measurement with a rubidium magnetometer (GPEX), the authors perform a Bayesian analysis of the PSD and find no robust signals, setting 95% CL limits: $g_{a\gamma\gamma} < 7\times10^{-10}\,\\mathrm{GeV^{-1}}$ and $\\epsilon < 2\times10^{-6}$ for masses between $3.5\times10^{-16}$ and $1.8\times10^{-14}\,\\mathrm{eV}$. Compared with prior geomagnetic searches, these bounds improve on the SNIPE-Hunt results for axions and tighten dark-photon constraints, demonstrating the viability of unshielded quantum sensors for ULDM searches and projecting that networks of such detectors could reach ~3 orders of magnitude better sensitivity with extended data taking.

Abstract

Ultralight dark matter candidates, such as axions and dark photons, are leading dark matter candidates. They may couple feebly to photons, sourcing oscillating electromagnetic signals in the Earth's conducting cavity formed between the ground and the ionosphere, providing detectable magnetic field signatures at wavelengths above the Earth's size. We carry out a project aiming to search for new physics using an unshielded high-sensitivity atomic magnetometer, termed the Geomagnetic Probe for nEw physiCS (GPEX). In this work, we report our first search for axion and dark photon dark matter, conducted in the desert of XiaoDushan in Gansu Province, China. Analysis of the collection of one-hour data shows no robust evidence for axion- or dark photon-induced magnetic signals. Correspondingly, we set the constraints on the axion-photon coupling with $g_{aγγ} < 7\times10^{-10}\, \mathrm{GeV^{-1}}$ and the dark photon kinetic-mixing parameter $ε< 2\times10^{-6}$ in the mass range $3.5 \times 10^{-16}\, \mathrm{eV} \sim 1.8 \times 10^{-14}\, \mathrm{eV}$. Our findings demonstrate the feasibility of using ground-based quantum magnetic sensors for ultralight dark matter searches. Future networks of such detectors operating over extended periods could improve the sensitivity by about three orders of magnitude.

Figures (6)

• Figure 1: Setup for our experiment. Away from the Dunhuang city 120 km, in the desert of XiaoDushan, we made our one-hour high performance measurement using the QTFM-B scalar atmoic magnetometer.
• Figure 2: PSD of the observed magnetic fields, $\hat{S}$ in green. and the standard deviation, $\sigma$ in blue, evaluated from 200 bins in the immediate vicinity on both sides excluding three center bins. We also marked the naive signal candidates with ${\rm SNR}\geq 2$ by circles in red.
• Figure 3: SNR for the frequency of interest. The horizontal solid lines is SNR= 2.
• Figure 4: Left: 95% CL limit on $g_{a\gamma\gamma}$ vs $m_a$ from GPEX, compared with SuperMAG Arza:2021ekqFriel:2024shg, SNIPE-Hunt Sulai:2023zqw, Eskdalemuir Taruya:2025zqlNishizawa:2025xka, CAST CAST:2017uph, and the quasar H1821+643 bound Reynes:2021bpe. Right: 95% CL limit on $\epsilon$ vs $m_{\gamma'}$ from GPEX, compared with SuperMAG Fedderke:2021aqoFedderke:2021rrmFriel:2024shg, SNIPE-Hunt Sulai:2023zqw, AMAILS Jiang:2023jhl, and the Leo T dwarf bound Wadekar:2019mpc. Prospective sensitivity for 10 magnetometers ($0.54~\mathrm{pT/\sqrt{Hz}}$, 1 month) is also shown.
• Figure S1: Six representative noise amplitude distributions from observed PSD at specific frequencies. The blue curves denote the best-fit $\chi^2$ distributions, along with the corresponding fitted dofs, scale parameters, and $p$-values from the goodness-of-fit test.