Quantum entanglement in attosecond ionisation

Two of the quantum phenomena at the very heart of the foundation of quantum mechanics, as it emerged about 100 years ago, were photoionisation (Einstein's 1921 Nobel prize for the discovery of the law of the photoelectric effect) and later on quantum entanglement, which led to the famous Einstein-Podolsky-Rosen (EPR) paradox and the puzzling concept of 'spooky action at a distance'.

The project proposed here joins these two fundamental concepts into a single research direction studying the formation and control of entanglement in photoionisation by ultra-short laser pulses. This theoretical endeavour emerges as very timely due to the recent developments in ultrafast laser technology that have opened the way to the experimental study of the very first few femtoseconds of the motion of electrons triggered by ionisation. A wide range of physical scenarios of photoionisation that previously could not be observed as they evolve in time, have become experimentally accessible to time-resolved spectroscopy performed with the new generation ultrafast light sources at both university laboratories and the major international laser facilities around the world. These scenarios encompass various regimes of the light-matter interaction, from the single- and multi-photon perturbative one to the strong-field non-perturbative one, as well as
various statistical properties of the ionising light, from fully coherent to stochastic light pulses.

By using quantum information and quantum state tomography approaches, we aim to discover new physical phenomena underpinned by quantum entanglement in ionising many-electron systems, occurring on the attosecond (10^{-18} of a second) time scale. As part of this research, we will study quantum entanglement between the photoelectron and the remaining ion, devise Bell-test experiments with photoelectrons to probe this entanglement and find out about the effect of measurement on the quantum many-electron dynamics. The proposed research programme will apply the world-leading theoretical and computational tools we developed to study the nature of
charge migration and other previously unexplored attosecond-scale processes in atoms and molecules, including the building blocks of biomolecules. These tools, originally based on Feynman's diagrammatic approach to quantum mechanical perturbation theory, are significantly
extended by us to describe the large-amplitude motion of the ionised electron.

As a result of our work, there will emerge the presently lacking picture of the effect of quantum entanglement and quantum measurement in photo-ionised many-body systems. The knowledge gained from this research will lead to a new level of understanding of the first moments in the electronic excitation of matter. Ultimately, this may lead to new ways to control the radiation damage processes, with direct implications on radiobiology and eventually on radiotherapy. For instance, it has been shown that electronic excitation in key molecular building blocks of matter is followed by a universal primary event - sub-femtosecond to few femtosecond migration of electric charge across nanometres. This
charge migration is expected to be extremely important for triggering the subsequent nuclear dynamics and therefore ultimately controlling the chemical change in what has become known as "attochemistry".