Attosecond physics: facing the wave-particle duality

Recent progress in generation and control of intense optical fields has given rise to isolated soft x-ray pulses with durations significantly below 1 fs. These constitute a tool of unprecedented temporal definition for attosecond physics-the study and manipulation of electronic motion on a time scale approaching the atomic unit of time. A key mechanism in such experiments is the welldefined momentum transfer between a quasi-free electron, released from an atom following irradiation by a short x-ray pulse, and a precisely controlled strong visible light field. The electrons' final kinetic energy thus sensitively depends on the timing of electron release with respect to the field oscillations and reveals the ejected electrons' confinement in time with sub-cycle, i.e. attosecond, resolution. Experiments resulting in electron emission of different durations can be interpreted in terms of a particle-like or wave-like electron, depending on whether the emission duration is considerably shorter or longer than the wave period of the probing light.