## Hi Hugo, you will defend your thesis soon, what is it about?

"In my thesis I use attosecond photoelectron interferometry. This is a measurement technique where we use a short laser pulse to eject an electron from an atom. The ejected electron behaves like a wave and can be described as a wave packet. We can measure the amplitude and phase of these electron wave packets, which helps us understand the dynamics of electron ejection.

Sometimes the electron can be prepared in a special state that cannot be described as a wave packet with a well-defined amplitude and phase. The electron is in a statistical mixture of states, and we do not know which until we observe it. In my thesis, we developed a method called KRAKEN to measure something called the density matrix of these mixed electrons.

The density matrix is a generalisation of the wave function and gives us information about the probability of finding the electron in different states and how they are connected to each other. With KRAKEN we can measure the electron density matrix and thus characterise both pure and mixed electrons.

By measuring the electron density matrix, we can gain a deeper understanding of how the electron behaves on ultra-short time scales, which can be important for the development of new materials and in photochemistry."

## Why did you choose this particular subject?

"It was largely a coincidence. I applied for and got a PhD position in attosecond physics with Anne L'Huillier as my supervisor. Anne was very generous in allowing me to explore what interested me. In discussions with my colleague David Busto and my supervisor Anne L'Huillier, we identified the limitations of the photoelectron interferometry protocol, RABBIT, and found that it could not characterise mixed quantum states. Having established that the density matrix of the photoelectron needs to be measured, it was like solving a puzzle to find a way to measure the density matrix."

## What is the most important result from your thesis?

"That it is indeed possible to measure the density matrix of a photoelectron created by the absorption of ultrashort laser pulses. Moreover, that in some specific cases, the entanglement between the photoelectron and the ion can be quantified using KRAKEN. This new measurement technique links two research areas, which at first glance may seem very different, attosecond science and quantum information science."

## What do you hope your thesis will lead to?

"In ultrafast physics, we study very fast processes at the atomic and molecular level. Decoherence involves a loss of quantum mechanical properties of an object, leading to more classical behaviour. Entanglement, on the other hand, is a quantum phenomenon where two objects become coupled in a way that their properties cannot be described separately.

Using the KRAKEN method, we can measure the decoherence of the photoelectron and, in some systems, quantify the entanglement between the ion and the photoelectron. Overall, I hope that KRAKEN will help increase our knowledge and understanding of ultrafast physics and lead to exciting advances in quantum mechanics and applications of quantum technology."

## What will you do when you finish the thesis?

"I have accepted a postdoctoral position at Lawrence Berkeley National Laboratory under the supervision of Professor Stephen Leone. I will try to transfer my expertise in coherence and decoherence in attosecond photoelectron interferometry to the experimental technique of attosecond transient absorption spectroscopy to study electronic coherence of semiconducting nanowires on ultra-short time scales."

## Why is it important to do research?

"The scientific method has been successful in gradually improving the standard of living through innovation of technologies. Therefore, it is meaningful to devote your creativity to solve problems that can help many. Additionally, it is interesting that using mathematics and physics it is possible to uncover hidden structures of reality, which to me is very mysterious."

## Dissertation

For the degree of doctor in physics, Hugo Laurellan has composed an academic thesis entitled:

"Attosecond photoelectron interferometry: from wavepackets to density matrices"

Hugo Laurell´s thesis is available at the Lund University Research Portal

Hugo Laurell defends his thesis on June 9, more information can be found in our calendar.