Coherent control of (non-)Hermitian mode coupling: tunable chirality and exceptional point dynamics in photonic microresonators

Steering Light on a Chip

Modern technologies from the internet to medical sensors all rely on tiny streams of light guided through microscopic circuits. This paper presents a new kind of on‑chip light circuit that can steer and reshape these streams with exceptional precision, opening doors to ultra‑sensitive sensors, compact optical computers, and devices that mimic how neurons process information.

A Tiny Racetrack for Light

At the heart of the work is a structure called the Dynamically Reconfigurable Unified Microresonator, or DRUM. You can picture it as a miniature racetrack for light, a ring carved into a silicon chip. Light can race around this track in two directions—clockwise and counterclockwise—while another straight “bus” road brings light in and out. Two side loops, called lobes, tap into the ring and send some light out and back in, letting the device carefully mix the two counter‑propagating directions. Each lobe contains built‑in heaters that can slightly warm the waveguides, which changes how light travels through them. By adjusting the electric power sent to these heaters, the researchers can independently control how strongly light traveling in one direction is converted into light traveling in the opposite direction, and how much phase delay is added along the way.

Tuning Between Two Kinds of Degeneracy

When waves share the same frequency, physicists say they are “degenerate.” In closed, lossless systems, such degeneracies are called diabolic points; in open systems that can lose energy, more exotic degeneracies called exceptional points arise, where not only the frequencies but also the shapes of the modes merge. The DRUM is designed to move smoothly between these regimes. By changing the strength and phase of the coupling in each lobe, the team maps out how the two resonant modes of the ring split or merge. They visualize this behavior as two curved energy surfaces that can touch or separate in three‑dimensional plots. Using measured transmission and reflection spectra, they show that the real device closely follows the predictions of a standard theoretical framework used for optical resonators, confirming that they can dial in almost any point on these energy surfaces.

Reshaping Resonances and Silencing Scattering

Because the DRUM controls how the two directions of light talk to each other, it can reshape each resonance—those sharp dips or peaks in transmission that mark where light is stored most strongly in the ring. By adjusting only the phase shifters, the team transforms a single, narrow resonance into a split doublet and back again, without changing how strongly light is coupled in and out. This lets them tune the effective sharpness, or quality factor, of a resonance well beyond what a similar but simpler ring could achieve with the same total losses. They also tackle a common nuisance in such devices: random backscattering from tiny imperfections in the waveguides, which normally mixes the two directions in an uncontrolled way. Using an optimization algorithm that drives the heaters, they arrange for the engineered coupling in the lobes to cancel this unwanted mixing. In this special configuration, known as a diabolic point, light travels around the ring in a single direction with no measurable reflection back to the input.

Creating One‑Way Light Flow

By pushing the device into a different operating regime, the researchers reach exceptional points where the two resonant modes fully merge but the device’s response becomes strongly directional. In one configuration, light injected from one side produces almost no reflection, while light from the opposite side is strongly reflected—essentially a one‑way mirror for specific wavelengths on a chip. The team quantifies this behavior with a “chirality” measure that captures which direction dominates. At the two exceptional points of the DRUM, this chirality reaches its extreme values, meaning the device achieves nearly perfect one‑way operation. By jointly tuning the heaters in the two lobes, they smoothly vary chirality from strongly one‑sided in one direction, through a symmetric state, to strongly one‑sided in the opposite direction, and they show that this behavior is stable and repeatable over many runs.

Why This Matters

To a non‑specialist, the key message is that the authors have built a compact silicon device that lets engineers “dial in” how light circulates, splits, and reflects on a chip, with real‑time, reversible control. Unlike earlier designs that could only access a few fixed operating points, the DRUM can move continuously between ordinary and exceptional behavior, cancel out unwanted scattering, and create highly directional responses on demand. This level of control over tiny light circuits is a powerful building block for future technologies, including ultrasensitive detectors that exploit exceptional points, reconfigurable optical logic for energy‑efficient computing, and neuromorphic hardware where light behaves in ways reminiscent of spiking neurons in the brain.

Citation: Aslan, B., Franchi, R., Biasi, S. et al. Coherent control of (non-)Hermitian mode coupling: tunable chirality and exceptional point dynamics in photonic microresonators. Light Sci Appl 15, 150 (2026). https://doi.org/10.1038/s41377-025-02176-3

Keywords: integrated photonics, microresonator, exceptional point, non-Hermitian optics, chiral light