Three Questions for Prof. Dr. Daniel Braun
This time, we ask Prof. Dr. Daniel Braun, professor at the Institute for Theoretical Physics at Eberhard Karls University in Tübingen, three questions. His research interests lie in the fields of quantum metrology, quantum information theory, quantum optics, quantum thermodynamics, mesoscopic solid-state physics, quantum chaos, nanomagnetism, and spintronics.
What is the focus of your research in the field of photonics and what originally attracted you to this field of study?
Most of my research is in quantum metrology with a focus on quantum optics, quantum imaging, opto-mechanical systems, and small gravitational effects. I got into this field by working on dissipative quantum chaos with the late Fritz Haake, one of the pioneers of superradiance. We needed a simple dissipation and decoherence mechanism to combine with the kicked top, and so superradiance was a natural candidate. And even though, by the end of the 1990s, it had been studied already for decades, we found that the same cavity-QED system also offered a huge decoherence-free subspace, as it was later called. That prompted my diving into the rapidly developing field of quantum information, where we found out how to do quantum computation in a decoherence-free subspace, and later into quantum metrology, by realizing that the decoherence-free subspace can give rise to very sensitive measurements when perturbed.
What are some of the most surprising or unexpected applications of photonics that you have encountered in your research or studies?
I continue to be in awe how sensitive one can measure different things using light, and in particular with opto-mechanical systems. LIGO was already mind-boggling when they managed for the first time to observe gravitational waves, but meanwhile they have pushed their sensitivities by roughly another 2 orders of magnitude. Another amazing example is the measurement of extremely small magnetic fields in alkali-vapor magnetometers: First you use circularly-polarized light to spin-polarize a gaz of vaporized metal. The spins precess a little bit in the tiny magnetic field, and then one uses light again to read-out that precession and infers the magnetic field from that. This has given rise to some of the most sensitive magnetometers. Or a final example: using the small amount of light scattered from macro-molecules to measure their mass. The sensitivity-limits of this "photometry" are pushed to smaller and smaller molecules, and the technique might at some point replace standard mass spectrometry.
What excites you most about the future of photonics, and where do you see your research taking you in the coming years?
Photonics is a very broad field with lots of physics in itself, and overlaps with other fields. I will continue to explore its metrological use, in particular for measuring small gravitational effects. We are very excited about the possibility of measuring the gravitational pull of the proton beam of the LHC at CERN with opto-mechanical sensors. This would allow testing general relativity in a totally new parameter regime, where the source of gravity is essentially almost pure kinetic energy rather than mass - and this in a controlled lab experiment! Further down the road it might even become possible to measure the gravitational pull of light itself, but this will still take years of development of powerful enough sources.
