Detecting Axion Dark Matter with an Organic Molecular Maser

TL;DR

Facing the challenge of detecting axion DM via axion–electron couplings in the μeV mass range, the work proposes a spin-based organic molecular maser as a quantum sensor. The method converts the axion-induced oscillating pseudo-magnetic field into a measurable microwave signal using a pentacene-doped p-terphenyl gain medium, achieving a direct laboratory constraint gaee ≈ 8 × 10^-6 GeV^-1 at m_a ≈ 6 μeV and 0.85 fT/√Hz sensitivity. It avoids ultra-strong magnets and cryogenics and is scalable to other spin systems, with potential CW operation and broadband extensions via Zeeman tuning and multi-material integration. This work broadens the DM search landscape into a readily deployable, room-temperature platform that can be extended to broader mass ranges and couplings.

Abstract

We present a novel quantum sensing approach to search for axion-electron interactions around the axion mass of 6 \mueV. In this region, laboratory searches are relatively scarce, and our direct experiment measuring the axion-electron coupling constant reaches the sensitivity of 8 \times 10^{-6} GeV^{-1}. The method, based on an organic molecular maser establishes a proof-of-principle for quantum-enhanced detection, with a corresponding magnetic field sensitivity of 0.85 fT/\sqrt{\rm{Hz}}. The methodology is generic and can be readily extended to other physical systems, further broadening its applicability in quantum sensing and dark matter searches.

Figures (4)

• Figure 1: Three-dimensional cross-sectional model of the axion detector. Pentacene-doped p-terphenyl (red) is embedded within a strontium titanate ring (transparent), enclosed in a shell made of oxygen-free copper (yellow), forming a sealed volume. The metallic enclosure provides electromagnetic shielding that blocks external electromagnetic noise photons (red sphere and arrows), while the axions (pink spheres and arrows) can penetrate the copper shell and interact with the photoexcited pentacene molecules. This interaction induces the emission of detectable microwave photons (blue sphere and arrows). Inset: Energy level diagram of the pentacene spin system. After optical excitation, pentacene forms an effective two-level system consisting of the $\rm{T}_{\rm{X}}$ and $\rm{T}_{\rm{Z}}$ sublevels. Interaction with the axion field induces stimulated emission of microwave photons.
• Figure 2: Magnetic field response of the pentacene maser. (a) Single-shot time-domain output signals of the pentacene maser response to microwave magnetic field under resonant ($\Delta\nu = 0$ MHz, blue) and near-resonant ($\Delta\nu = 0.9$ MHz, yellow) conditions. (b) At spin resonance, the output signal amplitude exhibits linear dependence on the amplitude of the applied microwave magnetic field. A linear fit yields a slope of $R = 0.126\,\rm{mV/pT}$, which characterizes the system responsivity.
• Figure 3: Workflow of dark matter detection. A single search cycle consists of four sequential steps: laser excitation of the sample, time-domain signal acquisition, and a subsequent waiting time because of laser repeat frequency 10 Hz. To ensure consistent of system conditions, the same procedure is repeated 1 million times without altering the experimental setup between cycles.
• Figure 4: Data analysis for axion detection. (a) Averaged time-domain signal obtained from 1 million measurement cycles. (b) Histogram of the time-domain data with a bin width of 0.5 $\mu$V. A Gaussian fit (yellow line) reveals that the signal follows a standard Gaussian distribution, indicating that the observed signal is white noise. (c) Fitted CDF of the Gaussian noise, yielding a noise standard deviation of $\sigma = 1.31(1)$$\mu$V. (d) Experimental constraint on the axion-electron coupling constant at the 95% confidence level within a mass range centered at $m_a$ = 5993.8 neV. This constraint is derived using the maser at a resonant frequency of approximately $\nu = 1.4493\,\rm{GHz}$.