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Interview: Prof. Jérôme Faist, ETH Zurich

Hello Jérôme, can you tell us about the current work of your group?

At my chair for Quantum Electronics at the ETH in Zurich, we work mainly in two fields: Quantum Cascade Lasers (QCLs) as direct sources in the THz-range and in the middle infrared. At the moment we focus on frequency combs based on QCLs which are the ideal source for spectroscopic applications. At the same time, we push the technology to work at room temperature.
We are also working on meta-materials which are the ideal tools to influence light-matter coupling and look into fundamental questions of quasiparticles and ultra-strong coupling.

Two of your recent papers reported on vacuum fluctuations of the electromagnetic field. What is the importance of understanding them in detail?

Vacuum field fluctuations are one of the most fundamental implications of quantum mechanics and a direct consequence of the uncertainty principle. Even in a pure vacuum there are finite fluctuations of the electromagnetic field. Although tiny, they manifest themselves in numerous effects, for instance triggering spontaneous emission of excited states in fluorescent light bulbs or LEDs. Investigating the fundamental characteristics of vacuum field fluctuations helps to understand those effects more in detail.

And what did you discover?

In “Electric field correlation measurements on the electromagnetic vacuum state” we observed the correlation of vacuum field fluctuations in different space-time volume depending on their separation in space and time. The result is the direct confirmation of the description of vacuum fluctuations as electromagnetic waves in quantum-theory. In “Magneto-transport controlled by Landau polariton states” we put a cavity around a Hall-bar and observed the direct current conductivity dependent on an external magnetic field and presence of the finite vacuum field and very weak terahertz illumination.

How is this different from conventional transport measurements?

For normal magneto-transport measurements, there is no cavity, only a Hall-bar. Illumination with electromagnetic fields does not have any effect on the conductivity. But when we couple the light-field with the cavity we can control the magneto-transport by illuminating the circuit with light.

So you put together tools from transport measurements and optics. How did you come up with this original idea?

You know, after my Ph.D. at EPFL, I went for a Postdoc to IBM in Rüschlikon where I learned about the transport applications. Because of my experience in both fields, I was hired by Federico Capasso at Bell Labs, where I worked on the QCL which has those two aspects as well - it was natural to put those fields together at some point.

And how did our MFLI help to perform the measurements?

With the current and the voltage inputs, the MFLI is ideally suited for transport measurements. We even use multiple of them at different sections of the same Hall-bar and read them out synchronously.

Does the effect have some practical application or is it only of academic interest?

In the first place, it is fundamental research but you can imagine the effect being used for ultrasensitive THz detectors. For me, fundamental research is always a driver for new and modern technologies.

Is this why you funded several companies?

I feel half a physicist and half an engineer. If only one half of me is involved, I am not happy. Even doing very fundamental research I always have in mind what can come out for practical applications. Helping start companies such as Alpes Lasers, IR Sweep, and Miro Analytical Technologies was a great possibility to bring technology from the lab into real applications.

And are you still involved in the companies?

Twenty years ago I was much more involved in Alpes Lasers compared to the start of the two others. But Alpes is now a mature company and does need me much less. And in IR Sweep and Miro Analytical, young people are driving the company and the business.

Prof. Jérôme Faist

Jérôme Faist, leading the Quantum Electronics chair at ETH Zurich and well known for his central role in the invention of the quantum cascade laser.

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