The search for axions sits at the intersection of solving critical problems in fundamental physics, including the strong C P problem in QCD, uncovering the nature of dark matter, and understanding the origin of the Universe’s matter-antimatter asymmetry. The measurement of axion mediated spin-dependent interactions offers a powerful approach for axion detection. However, it has long been restricted to regions outside the “axion window” due to a significant trade-off: the need to effectively suppress the magnetic leakage from highly polarized spin sources while simultaneously detecting subfemtotesla level exotic physics signals at sub-decimeter-scale distances. In this work, we report new experimental results on axion mediated exotic spin-spin interactions using an iron-shielded SmCo 5 spin source in combination with a specially designed self-compensation comagnetometer. Employing a composite shielding structure, we achieved a suppression of the magnetic field by up to 10 11 . This enabled us to establish new constraints on the coupling between electrons and neutrons, improving previous experimental limits by up to 10 000 times within the axion window. Furthermore, we also set the strongest constraints on the coupling between electrons and protons. The proposed method holds substantial potential not only for advancing the search for new physics beyond the standard model but also for enabling transformative applications in biological and chemical research.

Squeezed light, a prominent non-classical state of light, exhibits reduced quantum noise in one quadrature component below the standard quantum limit (SQL). The property enables quantum-enhanced precision measurements, surpassing the SQL in quantum sensing applications. This review comprehensively introduces the fundamental concepts, classifications, and experimental generation techniques of squeezed light. It further explores its pivotal role and recent advances in diverse quantum sensing domains, including interferometry, gravitational wave detection, magnetometry, force sensing, biomedical sensing, and quantum radar. The review covers theoretical foundations of squeezed states (including quadrature operators and classification schemes, experimental generation techniques in atomic ensembles, nonlinear crystals, and fibers), fundamentals of quantum sensing with squeezed light (from quantum noise theory to quantum-enhanced metrology), and quantum-enhanced sensing applications across the aforementioned domains. Finally, future challenges and opportunities in the field are discussed.

Optical rotation detection system (ORDS) utilizing quantum nondemolition measurement methods is widely applied in the field of quantum metrology and quantum information. However, the sensitivity of ORDS is limited by the uncertainty from optical-couple noise during the measurement of the atomic spin ensemble. In this study, we specifically analyze the mechanism of optical-couple noise caused by the fluctuations of probe light’s polarization in the modulated ORDS with a new model established to describe atomic spin precession in this particular condition. It is discovered that transverse electron-spin polarization errors are generated by the residual probe photon spin polarization in the ORDS, which results in extra coupling magnetic noise. In order to suppress this noise, a novel in-situ method is proposed that the resultant electron-spin errors are reduced by a specifically designed closed-loop system. The results are verified through the ORDS in a co-magnetometer. After zeroing the extra probe photon spin polarization, an angular sensitivity better than 1×10 ^-8 rad/Hz ^1/2 is achieved for frequencies higher than 5 Hz, demonstrating a probe background noise of 0.26 fT/Hz ^1/2 @14.5 Hz, approaching electronic noise and photon shot noise. With closed-loop control, the optical rotation bias instability is promoted by 4.2 times (from 107.9 to 25.7 μrad/h), and the angular noise of the ORDS is reduced by 2.4 times at 1-100 Hz. The measurement uncertainty of the realized ORDS nears the standard quantum limit, paving the road for long-term ultrasensitive measurements for new physics explorations.

Spin manipulation and detection of alkali-metal and noble-gas ensembles in spin-exchange-relaxation-free (SERF) comagnetometers are widely used in fundamental physical research, and have potential for inertial navigation. The dynamic behavior of spin ensembles is a vital characteristic that determines the applicability of comagnetometers, which significantly differs from various combinations of alkali-metal and noble-gas species. We comprehensively study the critical dynamics of strongly interacting ensembles in SERF comagnetometers with different species combinations. Different from the self-compensation point, a model of strong coupling point at which the dynamics is nonlinear because two ensembles influence each other strongly is established. We verify it by comparing the transient and steady-state dynamics of K-Rb-${}^{21}\mathrm{Ne}$, K-Rb-${}^3\mathrm{He}$, and K-${}^3\mathrm{He}$ comagnetometers with various input signals. The K-Rb-${}^{21}\mathrm{Ne}$ comagnetometer has a better potential for bandwidth and sensitivity in inertial measurement. Besides, we propose a critical point where the response rate of coupling spin ensembles is optimal among all polarization, and the critical dynamics is verified experimentally to be in agreement with theoretical simulations. The damping rate and precession frequency at the critical point can be more than 5 times larger than that at typical optimal spin polarization. Enhancement of spin-ensemble coupling strength improves the response rate of SERF comagnetometers and paves the way for efficient manipulation of noble-gas atoms via alkali-metal atoms in quantum information.

Ultrasensitive atomic comagnetometers have proven to be valuable tools for precise measurement and search for new physics. Besides, comagnetometers have potential for inertial navigation and quantum information. Despite their great potential, sensitivity improvement has been limited by different types of noise. To address this issue, the sources and magnitudes of noises are comprehensively analyzed and a suppression method is proposed for the most critical noise source, optical nonorthogonality. The optical nonorthogonality leads to critical transverse pumping effect, thus transverse light shift, and transverse effective magnetic noise. A direct correlation between the optical nonorthogonality and the electronic spin transverse relaxation rate is found, and a method for suppression of the optical nonorthogonality is proposed accordingly. Finally, the optical nonorthogonal noise is suppressed by 3.1 dB. Besides, an increase of 5.2 dB in the scale factor is achieved. With the combined effect of noise suppression and scale factor enhancement, the resulting inertial rotation sensitivity of $2.4 \times 10^{-8}$ rad/s/Hz $^{1/2}$ in the frequency range from 1 to 3 Hz is among the most sensitive comagnetometers, which opens up promising opportunities for new physics measurements, such as the search for ultralight dark matter and the fifth force.

High-sensitivity quantum sensors are a promising tool for experimental searches for beyond-Standard-Model interactions. Here, we demonstrate an atomic comagnetometer operating under a resonantly-coupled hybrid spin-resonance (HSR) regime to probe P-odd, T-even interactions. The HSR regime enables robust nuclear-electron spin coupling, enhancing measurement bandwidth and stability without compromising the high sensitivity of spin-exchange relaxation-free magnetometers. To minimize vibration-related noise from velocity-modulated sources, we implement a multistage vibration isolation system, achieving a vibration-related noise reduction exceeding 700-fold. We establish new constraints on vector-boson-mediated parity-violating interactions, improving experimental sensitivity by three orders of magnitude compared to previous limits. The new constraints complement existing astrophysical and laboratory studies of potential extensions to the Standard Model.