C HRISTIAN THEORY AND DESIGN OF QUANTUM K. A NDERSEN DEVICES IN CIRCUIT QED T The information technology of the future will contain a vast number of quantum ele- HEORY AND DESIGN OF QUANTUM DEVICES IN CIRCUIT ments with the promise of faster computers and more secure communication. The com- puter chips embedded into almost all technology today are made of silicon transistors and similarly will the quantum technology of tomorrow rely on mass produced quan- tum circuits. Circuit QED is one framework for building quantum circuits in which the information is encoded in superconducting quantum bits and operations are performed with the help of modern microwave technology. The aim of this thesis is to propose and describe devices that can be integrated into these circuits and, thus, enable new quantum technologies to emerge. A cornerstone in these devices is the so-called Josephson junc- tion, which works as a non-linear electrical component. It is shown in this thesis, that by combining Josephson junctions and microwave resonators, novel quantum effects, such as dynamics in the ultrastrong coupling regime and quantum-classical dynamics, can be explored. Moreover, this thesis describes a proposal for a microwave detector using current-biased Josephson junctions and a recent experiment which confirms the abilities of the device. Josephson junctions also play a key role in a proposed efficient approach to classical control of quantum devices working within the cryogenic environment of the quantum system. Finally, the thesis also considers the construction of desirable con- trollable couplings in circuit QED. Combined, these proposed designs offer a novel per- spective on the quantum information technology of the future using superconducting QED quantum circuits. D ISSERTATION AU AARHUS CHRISTIAN KRAGLUND ANDERSEN UNIVERSITY PHDDISSERTATION DEPARTMENT OF PHYSICS AND ASTRONOMY Theory and design of quantum devices in circuit QED Christian Kraglund Andersen P h D t h e s i s J u l y 2 0 1 6 Supervisor: Klaus Mølmer Department of Physics and Astronomy Aarhus University English summary The information technology of the future will contain a vast number of quantum elements with the promise of faster computers and more secure communication. The computer chips embedded into almost all technology today are made of silicon transistors and similarly will the quantum tech- nology of tomorrow rely on mass produced quantum circuits. Circuit QED is one framework for building quantum circuits in which the information is encoded in superconducting quantum bits and operations are performed with the help of modern microwave technology. The aim of this thesis is to propose and describe devices that can be integrated into these circuits and, thus, enable new quantum technologies to emerge. A cornerstone in these devices is the so-called Josephson junction, which works as a non-linear electrical component. It is shown in this thesis, that by combin- ing Josephson junctions and microwave resonators, novel quantum effects, such as dynamics in the ultrastrong coupling regime and quantum-classical dynamics, can be explored. Moreover, this thesis describes a proposal for a microwave detector using current-biased Josephson junctions and a recent experiment which confirms the abilities of the device. Josephson junctions also play a key role in a proposed efficient approach to classical control of quantum devices working within the cryogenic environment of the quantum system. Finally, the thesis also considers the construction of desirable controllable couplings in circuit QED. Combined, these proposed designs offer a novel perspective on the quantum information technology of the future using superconducting quantum circuits. i Dansk resumé (Danish summary) Fremtidens informationsteknologi kommer til at indeholder mange kvan- teelementer, hvilket vil medføre hurtigere computere og mere sikker kom- munikation. De computerchips der er inde i stort set al teknologi idag er bygget af silliciumtransistore, og på samme vis vil morgendagens kvan- teteknologi bygge på masseproducerede kvantekredsløb. Circuit QED er navnet på en en metode til at bygge disse kvantekredsløb, hvori kvantein- formation er gemt i superledende kvante-bits og hvor kvanteoperationer bliver udført ved hjælp af moderne mikrobølgeteknologi. Formålet med denne afhandling er at foreslå og beskrive enheder, der kan blive inte- greret i disse kredsløb og som dermed muliggør ny kvanteteknologi. En grundsten i disse enheder er den såkaldte Josephson junction, der virker som et ikke-liniært elektronisk element. Ved at kombinerer Josephson junctions med mikrobølgeresonatorer vil denne afhandling vise at det er muligt at udforske nye kvanteeffekter for eksempel ultrastærk kobling og kvante-klassisk dynamik. Derudover foreslåes det at en Josephson junction påtvunget en elektrisk strøm kan blive brugt som en mikrobølgedetektor og et nyligt udførst eksperiment viser at denne ide virker. Josephson junctions spiller også en afgørende rolle i en ny fremgangsmåde for klassisk kontrol af kvanteenheder, der bliver forslået i denne afhandling og som virker med samme teknologi som kvantesystemerne selv er implementeret med. Tilsidst kigger denne afhandling på hvordan forskellige ønskede koblinger i circuit QED kan kontrolleres. Tilsammen giver alle de forslåede design et nyt perspektiv på fremtidens kvanteinformationsteknologi implementeret med superledende kvantekredsløb. ii Preface In this thesis you find a presentation of research completed during my PhD studies performed at the Department of Physics and Astronomy at Aarhus University. The studies was funded by the Faculty of Science and Technology and further supported by the Danish Ministry of Higher Education and Science. The research was carried out between August 2011 and July 2016 under the supervision of Klaus Mølmer. Additionally, 5 months was spent during 2015 in the group of Alexandre Blais at University of Sherbrooke. The experimental data used for analysis in Secs. 3.2 and 4.3 was obtain by the groups of Michel Devoret at Yale and Evgeni Il’ichev at IPHT Jena respectively. On the next pages you will find a list of publications produced during my studies. The articles [1] and [4] are less related to the main topic of this thesis and, as such, will not be covered to any extend. The same goes for [15] which is an outreach article written in Danish. The papers [11–13] fits well within the scope of the thesis, but due to constrains of length these publications will not be included even though some of the general techniques used in these publications are presented in Chapter 2. The rest of the publications will be covered during the thesis, however not all details of all the papers will be presented and I encourage the reader to consult the papers themselves for additional details. As of writing, the papers [10–12, 14] are not published yet. I would like to thank Klaus Mølmer for his supervision during my studies. He has been a great supervisor; always ready with an impressive intuition and ready to challenge my ideas, but most importantly he has supported me greatly in developing and working out my own ideas. I am also very grateful to Alexandre Blais for the hospitality and our collaboration during my stay in Sherbrooke; his insight and expertise as well as his keen eye for details have taught me a lot. A thanks also goes to the all people I have worked with and discussed physics with over the years. There are too many people with whom I have enjoyed good iii iv discussions about physics, but in particular I would like to mention Jerome Bourassa, Samuel Boutin, Benjamin J. Chapman, Arne Grimsmo, The Jenses, Archana Kamal, Joseph Kerckhoff, Konrad W. Lehnert, Gregor Oelsner, Shruti Puri, Malte Tichy and Andrew C.J. Wade. Finally, a very big thanks goes to Eliska Greplova for supporting me in writing this thesis, for providing the drawing for the cover and for countless of valuable input and feedback. List of Publication [1] Christian Kraglund Andersen and Klaus Mølmer. Squeezing of collective excitations in spin ensembles. Phys. Rev. A, 86, 043831, (2012) [2] Christian Kraglund Andersen and Klaus Mølmer. Effective description of tunneling in a time-dependent potential with applications to voltage switching in Josephson junctions. Phys. Rev. A, 87, 052119, (2013) [3] Christian Kraglund Andersen, Gregor Oelsner, Evgeni Il’ichev, and Klaus Mølmer. Quantized resonator field coupled to a current-biased josephson junction in circuit QED. Phys. Rev. A, 89, 033853, (2014) [4] Malte C. Tichy and Christian Kraglund Andersen. Comment on ”Contextuality in bosonic bunching”. Phys. Rev. Lett., 113,138901, (2014) [5] Christian Kraglund Andersen and Klaus Mølmer. Circuit QED flip-flop memory with all-microwave switching. Phys. Rev. Applied, 3,024002, (2015) [6] Christian Kraglund Andersen and Klaus Mølmer. Multifrequency modes in superconducting resonators: Bridging frequency gaps in off-resonant couplings. Phys.Rev. A, 91,023828, (2015) [7] Christian Kraglund Andersen, Joseph Kerckhoff, Konrad W. Lehnert, Benjamin J. Chapman, and Klaus Mølmer. Closing a quantum feedback loop inside a cryostat: Autonomous state preparation and long-time memory of a superconducting qubit. Phys. Rev. A, 93,012346, (2016). v [8] G Oelsner, CK Andersen, M Rehák, M Schmelz, S Anders, M Grajcar, U Hübner, K Mølmer, and E Il’ichev. Detection of weak microwave fields with an underdamped Josephson junction. arXiv:1605.05935, (2016) [9] Christian Kraglund Andersen and Alexandre Blais. Ultrastrong coupling dynamics