An ordinary network of four optical waveguides is like an intersection without traffic lights: the signal wanders in all directions and mixes. But if you look only at some of the outputs, the picture changes. An illusion arises that the system has losses — though in reality energy doesn't disappear; it just goes into unobserved ports. This illusion is the key to the exceptional point.

An exceptional point is a state where two waves merge into one and start changing not smoothly but abruptly. Previously, such behavior required adding amplifiers or absorbers to the system. The authors showed that it's enough to properly choose phase delays in a passive network (without gain) and decide which ports to measure. A four-port network with two tunable phases allowed them to create two types of exceptional points: one with directional sensitivity, and another with linear splitting along two parameters simultaneously.

This approach is not tied to a specific material or wavelength. It works in optics, microwave engineering, and even acoustic circuits. And since control is achieved only through phases, the sensor can be retuned on the fly, like a radio receiver that tunes into a station without touching the antenna.