The hidden limit in light: intrinsic noise reshaping Brillouin metrology

Spontaneous Brillouin scattering is widely used to probe the mechanical and thermal state of matter, yet it has been assumed to be intrinsically stable. Jin and colleagues overturn this view by showing that spontaneous Brillouin light carries its own thermally driven noise floor. Their framework predicts-and experiments confirm-a universal upper bound of SNR = 1 under ideal detection conditions which can become even more restrictive than the conventional shot-noise limit in practical Brillouin systems. This discovery introduces a new fundamental limit to Brillouin-based sensing, microscopy and metrology.