Achieving high energy resolution in spin systems is important for fundamental physics research and precision measurements, with alkali-noble-gas comagnetometers being among the best available sensors. We found a new relaxation mechanism in such devices, the gradient of the Fermi-contact-interaction field that dominates the relaxation of hyperpolarized nuclear spins. We report on precise control over spin distribution, demonstrating a tenfold increase of nuclear spin hyperpolarization and transverse coherence time with optimal hybrid optical pumping. Operating in the self-compensation regime, our ${}^{21}\mathrm{Ne}\text{−}\mathrm{Rb}\text{−}\mathrm{K}$ comagnetometer achieves an ultrahigh inertial rotation sensitivity of $3\times {10}^{-8}\mathrm{rad}/\mathrm{s}/{\mathrm{Hz}}^{1/2}$ in the frequency range from 0.2 to 1.0 Hz, which is equivalent to the energy resolution of $3.1\times {10}^{-23}{\mathrm{eV}/\mathrm{Hz}}^{1/2}$. We propose to use this comagnetometer to search for exotic spin-dependent interactions involving proton and neutron spins. The projected sensitivity surpasses the previous experimental and astrophysical limits by more than 4 orders of magnitude.

Experimental searches for exotic spin-dependent forces are attracting a lot of attention because they allow to test theoretical extensions to the standard model. Here, we report an experimental search for possible exotic spin-dependent force, specifically spin-and-velocity-dependent forces, by using a K-Rb-²¹Ne co-magnetometer and a tungsten ring featuring a high nucleon density. Taking advantage of the high sensitivity of the co-magnetometer, the pseudomagnetic field from this exotic force is measured to be ≤7 aT. This sets limits on coupling constants for the neutron-nucleon and proton-nucleon interactions in the range of ≥0.1 m (mediator boson mass ≤2 μeV). The coupling constant limits are established to be $$|{g}_{V}^{n}|\,\le \,8.2\times 1{0}^{-11}$$∣ g V n ∣ ≤ 8.2 × 1 0 − 11 and $$|{g}_{V}^{p}|\,\le \,3.7\times 1{0}^{-10}$$∣ g V p ∣ ≤ 3.7 × 1 0 − 10 , which are more than one order of magnitude tighter than astronomical and cosmological limits on the coupling between the new gauge boson such as Z’ and standard model particles.

Advanced quantum sensor enables a search for spin physics beyond the standard model.

Typical weak signal search experiments rely on resonant effects, where the resonance frequency is scanned over a broad range, resulting in significant time consumption. In this study, we demonstrate an accelerated strategy that surpasses the typical resonance-bandwidth limited scan step without compromising sensitivity. We apply this method to an alkali-noble-gas spin system, achieving an approximately 30-fold increase in scanning step size. Additionally, we obtain an ultrahigh sensitivity of 1.29 fT ⋅ Hz^−1/2 at around 5 Hz, corresponding to an energy resolution of approximately 1.8 × 10⁻²³eV ⋅ Hz^−1/2, which is among the highest quantum energy resolutions reported. Furthermore, we use this sensor to search for axion-like particles, setting stringent constraints on axion-like particles (ALPs) in the 4.5–15.5 Hz Compton-frequency range coupling to neutrons and protons, improving on previous limits by several-fold. This accelerated strategy has potential applications in other resonant search experiments.

Searches for the axion and axionlike particles may hold the key to unlocking some of the deepest puzzles about our Universe, such as dark matter and dark energy. Here, we use the recently demonstrated spin-based amplifier to constrain such hypothetical particles within the well-motivated “axion window” ($10\mu \mathrm{eV}–1\mathrm{meV}$) through searching for an exotic dipole-dipole interaction between polarized electron and neutron spins. The key ingredient is the use of hyperpolarized long-lived ${}^{129}\mathrm{Xe}$ nuclear spins as an amplifier for the pseudomagnetic field generated by the exotic interaction. Using such a spin sensor, we obtain a direct upper bound on the product of coupling constants $g_p^e{g}_p^n$. The spin-based amplifier technique can be extended to searches for a wide variety of hypothetical particles beyond the standard model.