A laser beam enters a crystal, and inside it a dance begins that lasts less than one millionth of a nanosecond. In this crystal, electrons cannot move freely because they repel each other strongly, like people crammed into a packed bus. Light knocks out one electron, and a pair forms: a doublon (two electrons on the same site) and a holon (an empty site). They are bound together, like dance partners.

When the doublon and the holon meet again, they annihilate, and the extra energy escapes as a flash of light. If the laser field is strong, such encounters happen many times during a single light oscillation, and high harmonics are born: light at frequencies that are multiples of the laser frequency. The scientists used two laser pulses of different frequencies to isolate the even harmonics. By measuring when each harmonic is born, they learned how far the pair had managed to fly apart. The farther the separation, the higher the frequency of the light, just as with a spring: stretch it harder and the pitch goes up.

But the dance is not perfect. The high harmonics weaken toward the end of the laser pulse because the pairs collide with other particles and lose their synchronization, like dancers being jostled in a crowd. This limits the ability to control such systems with light, but on the bright side, scientists can now track the motion of electron pairs within a single light oscillation.