Exponentially enhanced sensing through nonreciprocal light propagation

Non-reciprocity is a key resource for pushing the performance of photonic devices beyond the fundamental limits imposed by Lorentz reciprocity. We experimentally realize an optical sensor in which nonreciprocal light propagation enables the detection of weak perturbations with a signal-to-noise ratio (SNR) that scales exponentially with system size. Our approach encodes two coupled Hatano–Nelson chains -- realizing an effective bosonic Kitaev model -- within the resonant modes of an electro-optic frequency comb. Nonreciprocal propagation in frequency space is achieved via simultaneous phase and amplitude modulation of the intracavity field. We demonstrate the sensing of a weak modulation tone coupling the two chains, observing an exponential enhancement of the SNR over three orders of magnitude as the system is scaled to over 70 frequency modes per chain. These results establish a new regime of non-Hermitian sensing, highlighting how nonreciprocal dynamics can be harnessed to achieve fundamentally enhanced measurement precision in photonic systems.