The interaction of light with matter provides indispensable insight into the quantum mechanical world of atoms and molecules on their intrinsic time and length scale. Compared to the macroscopic world, these scales are extreme: about 10 fs for the motion of the nuclei, about 10 as for the motion of the electrons; 0.2 nm is the typical length of a chemical bond. A major objective in science is the control of the nanoscopic processes on their extreme scales, which remains a challenge. Based on the concepts of quantum mechanics, specially tailored light fields can be used to address this problem. Here, the electromagnetic wave-character of light is exploited. By shaping the amplitude, phase and polarization of the electromagnetic waves, fields can be sculpted that enhance certain quantum processes while suppressing others, resulting in a net control of the system. The prerequisite for this type of quantum control is the ability to shape the electromagnetic field of ultrashort laser pulses with durations of just a few femtoseconds. Such pulses enabled scientists for the first time to trigger and control the atomic and molecular processes on their natural time scale.

In the visible range of the spectrum the spectro-temporal shaping of ultrashort laser pulses is a mature technique. Potential applications can be found in physics, chemistry and material science, for instance in the control of chemical reactions, efficient qubit manipulation, the exploration of complex reaction pathways, and the emergence of new spectroscopy concepts. To date, corresponding concepts in the extreme ultraviolet (XUV) and X-ray regime are hardly explored. The short-wavelength domain provides a perspective to access shorter time and length scales. Extremely short laser pulses with attosecond duration are available in this range, and highly localized inner-shell electrons can be addressed at these photon energies. Thus, extending spectro-temporal pulse shaping to the short-wavelength regime promises the quantum control of matter on unprecedented short time scales and with chemical sensitivity.

In this study, a key step towards this development is achieved: the control of quantum dynamics by the shaping of XUV pulses is established. The concept is applied to a fundamental problem, that is the control of the quantum dynamics in a strongly driven few-electron system. To this end, helium atoms are irradiated with highly intense XUV pulses produced by the free electron laser FERMI, a large-scale facility in Trieste, Italy. If an atom (or any other quantum system) is exposed to intense fields, the energy levels of the electrons start to shift or even split up into multiple states. Under these conditions, the quantum dynamics of the system have to be described by coupled electron-photon states, so called “dressed states”. In the experiment, these hybrid states exist only in an ultrashort time window during the perturbation of the system by the light pulse. The ultrafast dynamics of these short-lived states are successfully controlled by shaping the phase of the XUV laser pulses (see figure). These results are a fundamental demonstration that quantum control using shaped laser fields can be indeed extended to the short-wavelength domain. This opens an exciting route towards the exploration and control of matter with unprecedented capabilities.