Tuning a string to the right note: the laser pulse is adjusted so that harmonics play in unison at kiloelectronvolt frequencies.

Visible light oscillates at hundreds of trillions of times per second. If you take a very short laser pulse and direct it into a gas, the gas atoms start re-emitting light at multiples of the frequency, much like a string that, when plucked, sounds not only its fundamental note but also overtones. These overtones are called high harmonics. For them to combine into a bright flash, the waves from different atoms must stay in step. This condition is called phase matching, and it is like an orchestra where all instruments must come in exactly on time.

The RIKEN group used a laser with a wavelength of about 2.7 micrometers, which is several times longer than visible light. The pulse was focused by a concave mirror with a focal length of 3 meters into a helium-filled tube 50 millimeters long. The gas pressure was about one atmosphere, not tens of atmospheres as in previous experiments. The weak ionization of helium and the broad beam at the focus allowed waves from different atoms to add coherently, as if each atom were a musician who hears the conductor and comes in exactly on their beat.

When the phase of the laser pulse's carrier wave was shifted, the X-ray spectrum changed noticeably. The spectral cutoff shifted by 100 electronvolts, and a continuous region up to 300 electronvolts wide corresponded to a flash lasting only 12 attoseconds. This is shorter than the time it takes an electron in an atom to make one orbit around the nucleus (24 attoseconds).

Such a tool is needed to film the motion of electrons in metals. When a magnetic material demagnetizes or spin is transferred, these processes last attoseconds. Now physicists have a stroboscope capable of freezing such motion. And like a musician who has finally tuned the string, researchers can play the first note in a new symphony, a symphony of electron orbitals.