Light Splits Evenly: Geometry Instead of Mirrors express
Light from a single bulb usually spreads in all directions, and catching it in equal portions across several channels is quite a puzzle. In the new work, light is split into nine identical beams without a single lens or mirror. Instead, there's a glass plate with etched waveguide paths, along which light naturally flows in the desired way.
The secret is that light in such waveguides behaves not like a stream of particles but like a wave, and this wave has its own 'geometry'—a special property that depends on the shape of the path. By changing the shape of the paths, you can make the light split into strictly equal parts. It's as if a river, meeting a fork of nine identical channels, would naturally spread into them perfectly evenly.
Such precision is needed when signals from different beams must coincide down to the tiniest fractions of a wave. Today, complex control systems are used for this, but here geometry alone does the trick. This opens the way to miniature optical chips where light processes information like electrons in ordinary processors — only faster and without overheating.
Original abstract
as written by the authors arXiv:2602.04259
Light source is a foundational to photonic science and technology. However, a significant challenge remains in generating and distributing coherent light from a single on-chip source with high phase stability across multiple channels. Integrated lasers typically operate independently, and conventional splitters (e.g., multi-mode interferometers) do not guarantee the phase coherence required for advanced applications. Here, we report a purely geometric scheme for achieving massive equipartition of coherent light on a photonic chip by leveraging the geometric phases of a null space spanned by degenerate states with zero eigenvalue. The evolution of the null space maps to real-space rotation described by the special orthogonal group SO(N), thus enabling precise and scalable control over light distribution by engineering the system parameters. We experimentally realize up to one-to-nine equipartition of light on a waveguide array fabricated on a glass-based photonic chip. The framework can be upscaled for one-to-N light distribution. This work establishes a versatile and scalable platform for integrated coherent light sources, paving the way for integrated photonic applications such as quantum photonics and optical computing.
The glass microchip used to split light is smaller than a fingernail, yet it replaces an entire optical table with mirrors and lenses.
In the series 'The Expanse', light signals between ships and stations are transmitted through fiber optics — similar chips could make such communication more compact and reliable.