Tuning a string to the right note: a laser pulse is adjusted so that its harmonics sound in unison at kilo-electron-volt frequencies.

When you pluck a guitar string, it doesn't sound just one note but several at once: the fundamental tone and overtones. Gas atoms behave similarly when a short laser pulse passes through them: they re-emit light at multiples of the original frequency. These frequencies are called high harmonics. If all atoms emit in concert, like musicians in an orchestra following a conductor, the flash is bright and extremely short.

Physicists at RIKEN directed a laser pulse into a 50-millimeter-long tube filled with helium. The gas was at a pressure of about one atmosphere, similar to the air in a room. The beam was focused by a mirror with a focal length of 3 meters, so the beam at the focus was wide, and the helium ionization was weak. This allowed the waves from different atoms to stay in step. When the phase of the laser wave was shifted, the X-ray spectrum changed, and the scientists obtained a flash lasting 12 attoseconds. One attosecond is one billionth of one billionth of a second. In that time, light travels a distance smaller than the size of an atom.

Such flashes are needed to see how electrons move in metals. For example, when a magnet loses its magnetization, electrons rearrange within attoseconds. Now scientists have a stroboscope that can freeze this motion. And like a guitarist who has finally tuned the string, researchers are ready to play the first note in a new symphony: the symphony of electron orbitals.