Nonreciprocal coupling enables tunable exceptional-point response in non-Hermitian circuits

Abstract

Exceptional points (EPs) exhibit characteristic square-root spectral responses to weak perturbations, with response magnitudes governed by configuration-dependent prefactors that are often relatively small. Modulating these prefactor while preserving the exceptional characteristics offers an additional degrees of freedom for engineering non-Hermitian sensing responses. Here, we report a nonreciprocal non-Hermitian circuit system with gyrator-meditated coupling that supports a continuous family of EP configurations. Unlike reciprocal coupling, which typically yields a unique EP configuration for balanced gain and loss, nonreciprocal coupling allows the directional forward and backward coupling strengths to be varied while maintaining their product and thus preserving the EP condition and yielding a library of EPs. This additional freedom enables continuous control of the frequency-splitting magnitude without modifying the square-root characteristic dependence on absolute perturbation strength. We show that the spectral splitting induced by a given absolute resistance perturbation can be enhanced by appropriately adjusting the coupling asymmetry. Theoretical predictions, circuit simulations, and experimental reflection-spectrum measurements consistently confirms this tunable response. Our results establish nonreciprocal coupling as an alternative approach for engineering and enhancing EP-induced perturbative responses without changing their characteristic branch-point scaling.