In integrated circuits electrons are set in motion by applying a voltage, which exerts a force (of the order of 1 electronvolt/micrometer) on the electrons inside the semiconductor chip. The electrons’ flow, i.e. the electric current, is steered by the strength and direction of this force, which are controlled by the voltage applied from an electric signal generator. In the fastest computer chips, electrons are accelerated and stopped several billion times a second, i.e. their motion is controlled within a fraction of a nanosecond on a length scale of a fraction of a micrometer.

Steering electrons bound to atoms or molecules imposes much more severe demands on the steering force. Electrons can travel atomic distances, e.g. on a molecular orbital, within tens to hundreds of attoseconds. To affect this motion, a force comparable to or larger than the atomic forces (~10,000 – 100,000 electronvolts/micrometer) must be applied and varied on an attosecond time scale. Consequently, the electric field that exerts this force must be a million times stronger and switchable a million times faster than the fields acting in the fastest electronic circuits. These requirements can only be met by light fields.
If the field strength becomes much larger than 1 million electronvolts/micrometer, valence electrons are not only knocked free from atoms, but subsequently accelerated to velocities close to the speed of light within one period of the laser field. The controlled force of few-cycle light with shaped waveform is capable of steering the motion of bound electrons on atomic scales, which is the key to the reproducible generation of isolated attosecond pulses from atoms. Once available at higher intensities, controlled light forces will launch relativistic electrons on longer trajectories, opening the door for electron acceleration and attosecond pulse generation from relativistic interactions.