Preface Ph.D. thesis Caspar van der Wal

       This thesis presents results from four and a half years experimental research on Josephson quantum circuits in the group of Hans Mooij at Delft University of Technology. In December 1996, I started as a Ph.D. student in this group in a new project entitled Quantum computation with solid state devices. The project aimed at investigating whether small Josephson tunnel-junction devices could be used as basic building blocks for a quantum computer. It was financially supported by the Dutch Foundation for Fundamental Research on Matter (FOM). For fabrication of the Josephson junction circuits the cleanroom facilities of the Delft Institute for Micro Electronics and Submicron Technology (DIMES) were used. At a later stage efforts were worked out in collaboration with the groups of Terry Orlando, Seth Lloyd and Leonid Levitov at MIT. The project grew with the arrival of a few more Ph.D. students and postdocs, when the Delft DIOC on Novel computation structures based on quantum devices and the European SQUBIT program were started.
       In 1996, the field of quantum computation was quickly gaining momentum after Peter Shor had discovered in 1994 that a quantum computer would be able to factorize large numbers exponentially faster than a classical computer. However, it was already clear at that point that realizing a large quantum computer would be very difficult, if not impossible. The scientific community was very uncertain about the questions whether, and how a large-scale quantum computer could be built. Since then, there has been a lot of progress in the field of quantum computation, but the basic questions about the feasibility of quantum computation are still very open today. This uncertainty has led to a wide variety of proposals for research on realizing a quantum computer. An important advantage of implementing a quantum computer in microfabricated solid-state structures is, that the step from a basic unit to a large-scale integrated computer would be much easier than for many other proposals. This thesis reports on research that investigates whether small superconducting tunnel-junction circuits can form the required basic unit for a solid-state quantum computer. In the late 1980's and first half of the 1990's, the group of Hans Mooij has contributed significantly to research on mesoscopic tunnel-junction circuits. This provided a good basis for experimental work that aimed at demonstrating and accurately controlling quantum coherent dynamics of such circuits. While this research was inspired by the goal to realize a quantum computer, the results are often also very interesting for fundamental research on the validity of quantum mechanics and decoherence. The results in this thesis are therefore not presented with a strong emphasis on quantum computation.

       What I present in the following chapters is the result of work that I did together with students, technicians and other colleagues. I like to use this preface to thank all the people that contributed to this work, and colleagues that created a pleasant and inspiring atmosphere to work in. I enjoyed working in the Quantum Transport group very much.
       First of all, I thank Hans Mooij, he has been a very stimulating Ph.D. advisor who gave me a lot of freedom. It was very motivating that he was as excited about our research as I was. I benefited and learned a lot from his incredible intuition, when judging which long-term research goals were particularly interesting. During my Ph.D. work, Kees Harmans became a second Ph.D. advisor for me. With a lot of pleasure I recall many interesting discussions while working together on the new fridge. They were very important for creating a link between ideas about quantum physics, and getting results on the oscilloscope.
       I enjoyed being the advisor of five M.Sc. students: I like to thank Hans Cool, Paul Kuiper, Alexander ter Haar (now a Ph.D. student in our team), Thomas Ooms and Stijn Steenbrink for their friendship an hard work. I also thank Frank Wilhelm, a theorist who had the courage to work as a postdoc in our experimental group, for learning me a lot while working out questions about the influence of the measurement process in our experiments. Many other colleagues in Delft contributed to this work with help, critical questions, stimulating discussions, and being friends during and after work. I like to mention Leo Kouwenhoven, Peter Hadley, Herre van der Zant, Ton Wallast, Alexander van Oudenaarden, Milena Grifoni, David Dixon, Pieter Heij, Hannes Majer, Adrian Lupascu, Irinel Chiorescu, Tjerk Oosterkamp, Sander Tans, Henk Postma, Arnold Storm, Liesbeth Venema, Alberto Morpurgo, Sara Cronenwett, Lucas van Gorkom, Allard Katan, Marlies Goorden, and the visiting researchers Kostya Likharev, Michel Devoret, Hirotaka Tanaka, Yoshihiro Shimazu and Lin Tian.
       The support from several technicians in our Department has been indispensable for getting results. In particular, I like to mention Raymond Schouten. He deserves a lot of credit for designing and building the low-noise electronics that were a crucial part of the measurement setups. Wim Schot and Willem den Braver have provided a steady supply of liquid helium and nitrogen, and contributed significantly to setting up the new sorption-pump dilution refrigerator. Due to Bram van der Enden I never learned how to wire-bond my own sample, and he made sure that the setup for evaporating thin layers was running. I also like to thank Leo Lander, Bram Huis, Jan de Looff (stycast tricks) and Leo Dam for their work and advice on various occasions. I am indebted to the people at DIMES, in particular to Emile van der Drift, Anja van Langen, Arnold van Run and Bert de Groot, for support and advice on sample fabrication.
       In our collaboration with MIT, I worked a lot with Terry Orlando. I learned a lot from his insights in Josephson physics, and like to thank him for all his help. During my visits to MIT, I enjoyed working with the members of the Orlando group, and I am very grateful for their hospitality. I also thank Seth Lloyd and Leonid Levitov at MIT for their time and interest in my research. In the Netherlands, I learned a lot on frequent visits to Philip Stamp in Utrecht, and the NMR group of Jan Schmidt in Leiden. I also like to thank Jan Schmidt and Jos Disselhorst here once more for lending microwave material to us. While traveling, I had discussions with Yasunobu Nakamura, David DiVincenzo, Daniel Esteve, Orly Alter and Yoshihisa Yamamoto, that were of great value to me.
       It has been a pleasure for me to experience that not only physicists, but also friends and family can get excited about what happens at a few milliKelvin. Most notably, I like to thank my parents for always following my work with great interest. I also like to thank my partner Annet Jantien for her support. When recalling at this moment the years of my Ph.D. research, the strongest memory is that you have always been my best friend.

Caspar van der Wal
Delft, June 2001