Christine Falter

The development of a versatile and scalable platform for quantum components has the potential to fundamentally revolutionise everyday IT technologies. The excellent optical properties of a material platform based on the compound semiconductor zinc selenide (ZnSe) and its possibility for isotope stabilisation are proving to be especially promising.

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Christine Falter on an excursion by bike in Washington D.C.

Being a doctoral candidate in the working group under private lecturer Dr Alexander Pawlis at the Forschungszentrum Jülich gives me the opportunity to make a significant contribution to developing this key technology.

My working group has for several years been cooperating in this respect with Professor Edo Waks, whose group at the University of Maryland specialises in spectroscopy involving single-photon emitters. My DAAD-Stiftung funding enabled me to visit the Waks group this summer for three months and conduct exciting measurements on ZnSe samples produced at the Jülich. Detailed discussions with Professor Waks’s doctoral candidates prior to my stay meant that I was able to hit the ground running with the optical measurements in Maryland.

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Christine Falter during her free time

Even in the first weeks, we were therefore able to generate the spin states of our emitter system in the magnetic field and observe fluorescence under resonant optical excitation. As a next step, we subsequently focused on investigating the system dynamics in time-resolved measurements under resonant excitation.

These measurements are technically demanding and for me to a large extent new, since in my previous work I had sooner concentrated on basic optical characterisation under continuous overhead excitation. I therefore had the opportunity to learn about many new experimental techniques, such as the generation of different pulse sequences to excite a sample, using polarisation filtering to effectively suppress the reflected laser beam and working with photo detectors based on super-conductive nanowires.

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The laboratory

Our experimental work was accompanied by intensive weekly discussion with Professor Waks in which we made good progress in finding a coherent model for the dynamics of our system. After my return to Germany, we are now working on a joint publication of these results. We began to prepare further experiments towards the end of my stay.

This involved us firstly extending the existing optical set-up for more complex manipulations of our qubits’ spin state and we started on a second set-up in preparation for measuring at even lower temperatures. The core of this set-up is a dilution refrigerator with optical access that makes it possible to achieve temperatures down to only a few hundred mK.

What also makes our collaboration particularly interesting is the international nature of the working group and the wide diversity of our backgrounds. In addition to American students, I primarily collaborated with students from China, Iran and India. I’m convinced that this very internationality and our differing perspectives made a decisive contribution to the success of our experiments.

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Washington D.C., at the Capitol

The University of Maryland also has a lot to offer besides the research activities. This includes a huge sports and recreation centre with a number of Olympic-sized swimming pools, an on-campus farm and ice cream factory, and the opportunity to watch matches involving the Maryland Terrapins – the university’s football team. I especially enjoyed the organised excursions in the area surrounding the university. Trekking along the banks of the Potomac River or paddleboarding in Annapolis not only offered a great balance to the daily laboratory work, it also provided the opportunity for interaction with students from other research groups and disciplines.

The University of Maryland does not offer doctoral candidates accommodation on the university campus itself. I was however able to use the university’s ‘Off-Campus Housing Portal’ to quite easily find a room to rent in a house near the campus. I immediately got on well with my American housemates and we frequently ate together in the evening or arranged games evenings and movie nights.

I’m also extremely grateful that they were happy to give me a lift in their car, which made a lot of everyday things like shopping so much easier. I also noticed that ‘small talk’ is a lot more prevalent in the USA than in Germany. It wasn’t unusual for me to have pleasant conversations at the supermarket till, when locking up my bike or in the metro, which often meant that I left with specific tips for excursions.

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The farewell party of the parkrun friends

Even on my first weekend in College Park, I serendipitously came across the so-called park run – a free of charge 5 km running event organised each Saturday morning by volunteers and which regularly had more than 300 participants. My weekly participation not only improved my fitness, it also enabled me to meet all sorts of interesting personalities – from pupils at the local high school to the mayor of College Park – and gave me so many varied insights into the American culture and way of life.

I was also touched by the fact that my park run friends arranged a small leaving celebration at the end of my stay for which everyone contributed some food. This group too was very international, which was reflected in the diversity of dishes – which included everything from Japanese fish cake stew to Brazilian fudge balls. I’m certain that a good number of these friendships will persist beyond my stay. I’m also pleased to be able to continue with my new-found hobby back in Germany, since there is also a weekly park run that takes place in my home city of Aachen.

Mane Kaladzhian

The laboratory (here the one in Jülich) was essential for Christine Falter's research

I was surprised that the people in College Park are not so heavily reliant on cars as I had initially feared. It’s therefore reasonably straightforward to reach the centre of Washington D.C. by bike or metro in less than an hour. This meant that during my stay I was able to join with colleagues in visiting a number of the free to enter museums in the American capital.

I was particularly impressed by the Air and Space Museum, where among other things you can admire Neil Armstrong’s original Apollo 11 space suit, and by the National Gallery of Art, whose modern extension in Washington is hard to miss; especially given its statue of an oversized blue rooster by German sculptor Katharina Fritsch. One of my personal highlights in Washington was the very rousing Labor Day Concert performed by the National Symphony Orchestra, which takes place each year on the lawn of the US Capitol that is brightly illuminated at night.