In a dark room, an infrared laser flashes, and a speck of black K3C60 powder conducts electricity without resistance for a split second. The light strikes at 10 terahertz — a thousand times faster than a microwave oven. The effect is a hundred times stronger than at other frequencies. Why this particular frequency? Scientists built a computer model and found the answer.
Electrons in the material usually move individually, but during superconductivity they pair up, like dancers holding hands. Light at 10 terahertz acts like music that makes singles find a partner. The first photon kicks an electron out of the lineup, and the second photon helps it pair with another electron. This two-step path is allowed by the crystal's symmetry and works far more efficiently than single-photon absorption.
The model showed that the resonant frequency depends on the size of the crystal lattice. The larger the piece of material, the lower the frequency at which the enhancement occurs. For a small cluster of 14 atoms, the calculation gives 30 terahertz, while for a real crystal it's right around 10 terahertz. This matches the experiment. The scientists tested different electron interaction strengths and found that the effect is strongest at moderate coupling — not too weak, but not too strong either.
The result matters beyond K3C60. It shows that light can directly control electron pairing in a whole class of materials. These are called Hubbard materials, and they appear in many superconductors. Perhaps in the future, devices will switch superconductivity on with a short laser pulse.
The 10 terahertz frequency lies in the far-infrared range. Such radiation is absorbed by water vapor in the air, so experiments are conducted in vacuum or in a dry nitrogen atmosphere.
In Arthur C. Clarke's novel "The Light of Other Days," light is used to observe the past, but here it literally switches the quantum state of matter.