Christian Frydendahl

Project title

Quantum Control of Emission in Valleytronic Materials

What is your project about?

Light has a hidden property called spin: a photon can rotate either to the right or to the left around itself – just as our two hands mirror each other without being identical. This 'chirality' is a key to many new quantum technologies such as quantum computers and quantum encryption, because light with a particular spin can control the state of other quantum objects like atoms and electrons. The problem is that almost all materials emit a 50/50 mixture of right- and left-handed light. If you want to use only one kind, it requires large optical systems to filter out the rest. My project will solve the problem at its root in a new class of 2D semiconductors. By passing current through the material in a particular way, we can sort the charges inside a light-emitting diode, so that only light with precisely the spin we want is emitted. The result is a small, compact source of 'pure' chiral light – an important building block in the quantum technology of the future.

How did you become interested in your particular field of research?

For me, the most exciting thing about physics is its inherent breadth: physics is ultimately an attempt to understand the basic elements that make up our universe, and how their interactions build the reality we experience. I am constantly fascinated by how a handful of fundamental principles can explain such widely different phenomena. That is why my research field suits me so well – nano-optics lies at the intersection of solid-state physics and quantum optics, and here the same physics appears in new disguises again and again. At the same time, the phenomena we study are often directly applicable in sensors, light sources and other technologies. So even though much of my work is basic research, it is always anchored in concrete questions: "What makes a good sensor, and how can we use the physics behind it to make it better?"

What are the scientific challenges and perspectives in your project?

The central challenge is fundamental: how do you control light's spin already inside a material – before the light has even been created? Until now, it has only been possible to 'sort' the photons' direction of rotation afterwards, with large optical systems, and never electrically and directly at the source. In my project I exploit the fact that the charges in atomically thin 2D semiconductors carry a kind of internal direction. By passing current through the material in a particular way, I can gather charges with the same direction in one place, so that they emit only light with precisely the spin I want. An important obstacle is that the charges lose this direction quickly (they flip-flop between the two states) – therefore I couple the LED to nano-optical resonators that get the light emitted before the direction is lost. In the longer term, this very coupling to a resonator can also pave the way for an actual chiral laser source.

What is your estimate of the impact, which your project may have to society in the long term?

My research is basic research, and the concrete applications lie years into the future – but there is always a direction toward application. Compact sources of chiral light that can be built and scaled directly on a chip are one of the missing building blocks in what is called the second quantum revolution. If we succeed in getting that kind of light source to work in the long run, it will become easier to move quantum technology out of the laboratory and make it practical: computers that can solve problems our current ones cannot, communication secured by the laws of physics rather than by codes, and sensors with unprecedented sensitivity. In the long term, this could have significance for everything from medicine and materials development to data security.

Which impact do you expect the Sapere Aude programme will have on your career as a researcher?

Sapere Aude marks the transition from being part of others' research groups to standing on my own feet as an independent research leader. It now gives me the opportunity to gather the experience and abilities I have built up through a series of postdoc positions, in order to form a new research direction in Denmark – based on my own ideas, which I have developed over the years. The grant gives this new group a favourable start in the form of starting resources. Concretely, the grant also means that I can bring my national and international network together in lasting collaboration. Just as importantly, I gain the framework to pass the knowledge that is built up on to the next generation of researchers – among other things by training a PhD student who can themselves help carry Danish quantum research further in the years to come.