Physics-paired stimulated Raman scattering microscopy enables label-free phenotyping of lipid droplets 3D motility in live cells

High-speed volumetric stimulated Raman scattering (SRS) microscopy offers unique capabilities for label-free chemical imaging in living systems, yet its performance is fundamentally constrained by the trade-off between imaging speed and signal-to-noise ratio (SNR). At the short pixel dwell times required for three-dimensional dynamic imaging, photon-limited detection leads to severe noise that cannot be effectively mitigated by existing denoising approaches, owing to the lack of ground truth data and temporally redundant measurements in live-cell conditions. Here we present PHYSIQ, a physics-paired in-phase and quadrature SRS imaging framework that fundamentally redefines data acquisition for noise-limited optical microscopy. By exploiting the intrinsic quadrature nature of heterodyne detection, PHYSIQ-SRS simultaneously acquires two spatially co-registered and temporally near-synchronous SRS image channels with statistically independent shot noise. This physics-paired measurement enables fully self-supervised Noise2Noise restoration without requiring ground truth or temporal redundancy. The implementation integrates dual-channel lock-in detection with defocus-corrected spatial co-registration and controlled temporal offset, establishing a robust and generalizable strategy for generating unbiased training pairs directly from physical measurements. This innovative approach achieves an SNR enhancement of ~12.5 dB while preserving quantitative Raman contrast, effectively overcoming the conventional speed-sensitivity limitation in volumetric SRS microscopy. The improved performance enables video-rate volumetric imaging and label-free 3D tracking of lipid droplets (LDs) in living cells. Using this capability, we uncover that LD dynamics are governed by discrete motility states with condition-dependent transitions, including spatial redistribution under nutrient perturbation, selective suppression of long-range transport upon glycolytic inhibition, and phase-dependent reprogramming during mitosis. PHYSIQ-SRS establishes a new paradigm of physics-enabled self-supervised imaging, providing a general solution to shot-noise-limited detection in laser-scanning microscopy. This advance opens new opportunities for high-speed, label-free volumetric imaging and quantitative investigation of live-cell biology, metabolic phenotyping, developmental imaging, and biomedical discovery.