Usually, light does not turn an insulator into a conductor: that requires powerful laser pulses that break the material's structure. But in twisted bilayer graphene at the magic angle, things are different. Here, a weak beam of photons with a wavelength like that of far-infrared radiation heats the electrons, and that is enough to destroy the insulating state.

The electron system in this material has a tiny heat capacity, like a pot of water that boils from a single match. The heating suppresses the correlated gap (the energy barrier that prevents electrons from moving freely), and the material abruptly becomes a metal. The resistance drops by orders of magnitude: this is not a gradual transition but a sharp contrast, like the click of a switch.

Such a phase transition gives rise to a sensitive photodetector: the output voltage changes by millivolts when absorbing just a nanowatt of radiation. The mechanism resembles that of superconducting bolometers, but in reverse: there, heating destroys superconductivity; here, an insulator becomes a conductor. The device is robust to magnetic fields of several tesla and works over a wide range of wavelengths.