# Noise-resilient exceptional point sensing with immunity to undesired perturbations

## TL;DR

This paper introduces a new sensing method using nonresonant exceptional points that are highly sensitive yet resistant to noise and local disturbances.

## Contribution

The novelty lies in using nonresonant exceptional points in metamaterials for robust and ultrasensitive sensing.

## Key findings

- Nonresonant EPDs show sublinear reflectance variation to global perturbations.
- Sensitivity is protected from noise and local disturbances in experiments.
- The method enables ultrasensitive sensing with immunity to cavity imperfections.

## Abstract

Exceptional point degeneracies (EPDs) are non-Hermitian singularities where K eigenvalues and their corresponding eigenvectors coalesce. When a small perturbation is induced, the eigenvalue detuning from an EPD follows a Kth-root sublinear expansion, which provides a means of enhancing the sensitivity (frequency shift) of resonant-based sensors. On the downside, resonant-based sensors are susceptible to cavity imperfections, local mechanical disturbances (temperature variations, vibrations), and other experimental uncertainties. Here, we overcome this problem by experimentally implementing passive periodic microwave metamaterials with nonresonant EPDs (NR-EPDs) occurring in their Bloch spectrum. We demonstrate a sublinear variation of the reflectance near NR-EPDs to a specific class of (global) perturbations and propose its usage for ultrasensitive sensing that is immune to undesired (local) perturbations. The sensitivity is shielded from technical or fundamental noise that typically degrades the signal-to-noise performance of resonant EPDs.

Nonresonant exceptional points enable ultrasensitive sensing that is robust to noise and immune to undesired local imperfections.