Ultralight axion-like particles (ALPs) are well-motivated dark matter candidates introduced by theories beyond the standard model. However, the constraints on the existence of ALPs through existing laboratory experiments are hindered by their current
sensitivities, which are usually weaker than astrophysical limits. Here, we demonstrate a new quantum sensor to search for ALPs in the mass range that spans about two decades from 8.3 feV to 744 feV. Our sensor makes use of hyperpolarized long-lived nuclear spins as a pre-amplifier that effectively enhances coherently oscillating axion-like dark-matter field by a factor of >100. Using spin-based amplifiers, we achieve an ultrahigh magnetic sensitivity of 18 fT/Hz$^{1/2}$, which is significantly better than state-of-the-art nuclear-spin magnetometers. Our experiment constrains the parameter space describing the coupling of ALPs to nucleons over our mass range, at 67.5 feV reaching $2.9times 10^{-9}~textrm{GeV}^{-1}$ ($95%$ confidence level), improving over previous laboratory limits by at least five orders of magnitude. Our measurements also constrain the ALP-nucleon quadratic interaction and dark photon-nucleon interaction with new limits beyond the astrophysical ones
The nature of dark matter, the invisible substance making up over $80%$ of the matter in the Universe, is one of the most fundamental mysteries of modern physics. Ultralight bosons such as axions, axion-like particles or dark photons could make up mo
st of the dark matter. Couplings between such bosons and nuclear spins may enable their direct detection via nuclear magnetic resonance (NMR) spectroscopy: as nuclear spins move through the galactic dark-matter halo, they couple to dark-matter and behave as if they were in an oscillating magnetic field, generating a dark-matter-driven NMR signal. As part of the Cosmic Axion Spin Precession Experiment (CASPEr), an NMR-based dark-matter search, we use ultralow-field NMR to probe the axion-fermion wind coupling and dark-photon couplings to nuclear spins. No dark matter signal was detected above background, establishing new experimental bounds for dark-matter bosons with masses ranging from $1.8times 10^{-16}$ to $7.8times 10^{-14}$ eV.
