Ordinary light is a stream of independent particles: photons fly through each other without noticing their neighbors. But in superconducting circuits cooled to near absolute zero, the situation changes. There, photons travel along narrow metal tracks, and if tiny elements called Josephson junctions are embedded in these tracks, light begins to 'feel' itself. Each photon slightly distorts the path for the next one, like people walking in single file, each stepping on the same springy floorboard in turn.

The authors of the new work calculated that with realistic parameters for such lines, the photon interaction can be amplified so much that they stop behaving like independent particles. Instead, they form a special state — a Tonks-Girardeau gas. In this state, photons mimic fermions: particles that cannot occupy the same place at the same time. Light seems to become 'harder' and acquires an internal structure, like a queue of people each keeping their distance.

The practical value of the idea is that it can be tested right now: one only needs to measure how light transmitted through such a line is distributed over time. If the photons have indeed become 'fermionized,' characteristic dips will appear in this distribution — signatures that the particles avoid each other. Such a platform could become the basis for quantum simulators where light models the behavior of complex materials, and for new types of quantum computation.

The idea of making light behave like a solid is not new, but previously it required bulky resonators. Here, everything happens in a propagating geometry — light is not trapped in a box but moves along the line, like water in a trough. And if the trough is made correctly, the water suddenly stops flowing as a continuous stream and instead flows in discrete 'portions' that jostle each other with their elbows.