Timing tunnelling to attosecond precision
According to the laws of quantum physics, electrons bound to atoms can penetrate through a potential wall confining their location like a wave. The wave may reach the other side of the potential without climbing over it. The question of how long it takes an electron to pass through the barrier has been the subject of theoretical debate since the early days of quantum physics. In 2008, researchers at the Swiss Federal Institute of Technology in Zurich (ETHZ) manipulated helium atoms with a laser pulse to lower the barrier to ionisation. This helped one of the electrons to tunnel out easily, and they could for the first time measure the tunnelling time. They proposed the 'Ultrafast lasers and attosecond dynamics' (ULAD) project to develop a solid theoretical framework for their experiments. During the course of the ULAD project, scientists compared predictions of the main competing theories and applied a new approach to calculating tunnelling times. Specifically, the probability distribution of tunnelling times was estimated with the use of Feynman's path integrals. This approach led to results that are in good agreement with the experimental data. ULAD scientists applied adiabatic and non-adiabatic models to calculate the tunnelling time from experimental measurements. They delved deep into the incompatibility of the theoretically derived tunnelling times with experimental measurements. This allowed them to gain a deeper theoretical understanding of the physical process behind electrons traversing across a potential barrier. The results of the ULAD project have been described in six scientific papers published in eminent peer-reviewed journals: three Physical Review Letters, Optica, J. Phys. B (recognized as J. Phys. B Highlight of 2013) and New Journal of Physics (recognised as a highlight of 2013 across all IOP journals). The research work also received significant attention at international conferences where it was presented. Already, further experiments have been planned at ETHZ to validate the ULAD project's findings. Resolving questions arising from experimental results will give physicists deeper insight into the structure of atoms as well as the ionisation process itself.
Keywords
Tunnelling, attosecond, electrons, quantum physics, ultrafast lasers
