Manipulating quantum paths for novel attosecond measurement methods

Sources of attosecond-duration light pulses provide the fastest time resolutions available today for observing ultrafast phenomena in atoms, molecules and condensed matter. The measurement of such pulse durations is challenging because the spectrum lies in the vacuum ultraviolet or soft X-ray range. Two classes of pulse duration measurements now exist; they are classified according to whether the measurement is performed in the generating medium or in a second medium. The first measurement class is called 'in situ' and depends on gently perturbing the electron responsible for attosecond pulse formation. The second measurement class, which we refer to as 'ex situ', takes place in a second medium in which photoelectrons are produced by the attosecond pulses; a synchronized laser field perturbs the photoelectron spectrum. This Review compares and contrasts these two approaches for measuring attosecond pulses.