Daniel Riedel: An award-winning scientist from the SNI PhD School

Dr. Daniel Riedel, a postdoc jointly in the groups of Professor Richard Warburton and Professor Patrick Maletinsky, is currently receiving one prize after the other for his doctoral thesis and an ensuing publication. As part of his doctoral dissertation, this former SNI PhD student drastically improved the quality of individual photons generated by a quantum system, successfully putting a 10-year-old theoretical calculation into practice.

Turning a hobby into a profession
Daniel Riedel began his doctoral dissertation at the SNI PhD School in June 2013. He was instantly attracted to the topic for a PhD position advertised by physics professors Richard Warburton and Patrick Maletinsky, which was intended to study particles of light (photons) emitted by nitrogen-vacancy centers (NV centers) in diamonds.

These NV centers are formed when two carbon atoms in the diamond lattice are replaced with a nitrogen atom and an adjacent vacancy. Due to the extremely high purity of the diamonds used, the electrons trapped in the vacancy behave like those of isolated atoms and can therefore be used for quantum information processing.

As Daniel Riedel had previously worked on vacancies in silicon carbide, their manipulation and use, as part of his degree dissertation at the University of Würzburg, applying to Basel seemed the obvious thing to do. “Accepting a position here was an easy decision,” he recalls. “I was quickly won over by the infrastructure and environment on offer in Basel, and I never regretted the decision. After all, my thesis felt more like a hobby to me than work,” he adds.

Greater light yield needed
His aim was to boost the photon yield of these NV centers without impairing their other positive properties in the nanofabrication process. The need for an improved photon yield stems from the large differences in refractive index between diamond and air, which causes most of the light emitted by the NV centers to be reflected at the interface. This light then remains inside the diamond, and only a small part of the emitted light reaches the outside.

Daniel Riedel began by considering the entire spectral region of the emitted light. Using a dielectric optical antenna, he was able to concentrate the photons in a specific direction and thus to capture them using a conventional lens. The antenna consists of a diamond membrane with a thickness of several hundred nanometers and containing individual NV centers. When this membrane is applied to the semiconductor material gallium phosphide (GaP), the boundary layer between the air, diamond, and GaP acts as an optical antenna. The reason why GaP is so well suited as a material is that it has a larger refractive index than diamond and is transparent in the spectral region where the NV center emits light.

Daniel Riedel studied the antenna’s radiation pattern for various layer thicknesses of the diamond membrane and found that it agreed excellently with an analytical model he had developed. “For very thin diamond layers, I was able to isolate individual NV centers and improve their light yield by an order of magnitude,” he explains.

Other problems in the pipeline
After this first part of the project, however, Riedel still had to deal with the poor quality of the photons. Only some three percent of all emitted photons had the necessary properties to establish quantum mechanical entanglement between two NV centers over large distances and therefore to use them to transmit information. In the last three years of his doctoral dissertation, Riedel turned his attention to solving this problem. “Progress wasn’t always fast,” he says. “At times, it took a great deal of stamina to keep on working with the same momentum.”

In the end, it was worth it. In his recent publication in Physical Review X, Daniel Riedel describes how he succeeded in raising the yield of suitable photons from three to almost 50 percent. He achieved this significant improvement by using an optical microresonator to boost the emission rate in a narrow frequency range. For this, Riedel placed a diamond membrane with a thickness of approximately 800 nanometers on a planar mirror that can be positioned with nanometer precision below a second mirror with curved depressions. Ten years earlier, a theoretical description had predicted that positioning NV centers in a resonator of this kind should increase the yield of photons. By precisely controlling the distance between the two mirrors, Daniel was able to couple various NV centers to the resonator in an optimum fashion, thereby increasing the emission rate of the desired photons so that they now make up almost 50 percent of the total emission.

A successful year
At the Swiss Nano Convention in June 2018, Daniel Riedel was presented with one of the Swiss Micro & Nanotechnology Network’s PhD Awards, which is sponsored by the Hightech Zentrum Aargau, for his publication in Physical Review X.

In March 2018, his overall doctoral dissertation had already earned him second place in the “dissertations” category of the Quantum Future Award from the Federal Ministry of Education and Research in Germany (BMBF) and the Center for Integrated Quantum Science and Technology (IQST). In his home town of Dinkelsbühl, Germany, he was also awarded the 2018 sponsorship prize by the Willi Dauberschmidt Foundation. Yet another recent addition to his list of awards, was the Early Postdoc.Mobility Fellowship from the Swiss National Science Foundation. The latter will enable him to embark on the next step in his career as a postdoc at the California Institute of Technology in Pasadena (USA).

The SNI is delighted for Daniel Riedel and would like to take this opportunity to congratulate him on these successes. For Daniel, the awards mark the successful completion of his prosperous and enjoyable time as a doctoral student at the SNI.

“I received excellent supervision from both of my supervisors, Richard Warburton and Patrick Maletinsky, and really enjoyed the close collaboration with both research groups. The workshops organized by the SNI Network and the NCCR QSIT were also real highlights for me and taught me to look beyond the boundaries of my own research.”