Decoherence in superconducting quantum circuits

Thumbnail Image
Journal Title
Journal ISSN
Volume Title
School of Science | Doctoral thesis (article-based) | Defence date: 2012-08-24
Checking the digitized thesis and permission for publishing
Instructions for the author
Degree programme
Aalto University publication series DOCTORAL DISSERTATIONS, 102/2012
Superconducting quantum devices have drawn the attention of physicists greatly in recent years, not only because they are one of the most favorable candidates for developing a solid state quantum computer, but also because they can be employed as test systems for understanding quantum mechanics at nearly macro scale. Superconducting quantum devices can be divided into two groups: the superconducting resonators (linear devices), which have equally spaced discrete energy levels, and the artificial atoms (non-linear devices) consisting of Josephson junctions which have unequally spaced discrete energy levels. By combining these two kinds of devices together, a solid state counterpart of optical cavity quantum electrodynamics (QED) known as circuit QED emerges. Experiments of cavity QED and quantum optics can be reproduced in either circuit QED systems or in bare superconducting artificial atoms. As an example, we have observed the Autler-Townes effect in a three-level artificial atom called phase qutrit. More than one decade has passed since the first experimental demonstration of superconducting two-level artificial atom (qubit). However, to build a practical quantum computer with thousands of superconducting qubits there is still a long way to go (we are at the stage of three qubits now). One of the main obstacles preventing us from scaling up a superconducting quantum computer, as well as some other kinds of quantum computers, has been decoherence, which is believed to be dominated by the low frequency noise due to the two-level system (TLS) fluctuators located inside the Josephson junction barrier. We have experimentally simulated the dynamics of a qubit longitudinally coupled to a randomly fluctuating TLS. A phenomenon known as motional averaging has been observed. In this thesis, theoretical models for decoherence in both resonator and artificial atoms have been established and used for explaining phenomena observed in the experiments performed in our laboratory. The phenomenon of decoherence in coupled bipartite systems has been also studied, and besides the already known entanglement sudden death phenomenon, stable entanglement (robust under decoherence) generated from an arbitrary initial state has been found in these systems.
Supervising professor
Kaivola, Matti, Prof.
Thesis advisor
Paraoanu, Sorin, Dr.
superconducting artificial atoms, circuit QED, decoherence, TLS, Autler-Townes effect, motional averaging, entanglement sudden death
Other note
  • [Publication 1]: Jian Li, K. Chalapat, and G. S. Paraoanu. Entanglement of superconducting qubits via microwave fields: Classical and quantum regimes. Physical Review B, 78, 064503, August 2008.
  • [Publication 2]: Jian Li, K. Chalapat, and G. S. Paraoanu. Enhancement of sudden death of entanglement for driven qubits. Journal of Low Temperature Physics, 153, 294, October 2008.
  • [Publication 3]: Jian Li, K. Chalapat, and G. S. Paraoanu. Measurement-induced entanglement of two superconducting qubits. Journal of Physics: Conference Series, 150, 022051, March 2009.
  • [Publication 4]: Jian Li and G. S. Paraoanu. Decay of entanglement in coupled, driven systems with bipartite decoherence. The European Physical Journal D, 56, 255, September 2009.
  • [Publication 5]: Jian Li and G. S. Paraoanu. Generation and propagation of entanglement in driven coupled-qubit systems. New Journal of Physics, 11, 113020, November 2009.
  • [Publication 6]: Mika A. Sillanpää, Jian Li, Katarina Cicak, Fabio Altomare, Jae I. Park, Raymond W. Simmonds, G. S. Paraoanu, and Pertti J. Hakonen. Autler-Townes Effect in a Superconducting Three-Level System. Physical Review Letters, 103, 193601, November 2009.
  • [Publication 7]: Jian Li, G. S. Paraoanu, Katarina Cicak, Fabio Altomare, Jae I. Park, Raymond W. Simmonds, Mika A. Sillanpää, and Pertti J. Hakonen. Decoherence, Autler-Townes effect, and dark states in two-tone driving of a three-level superconducting system. Physical Review B, 84, 104527, September 2011.
  • [Publication 8]: Jian Li, G. S. Paraoanu, Katarina Cicak, Fabio Altomare, Jae I. Park, Raymond W. Simmonds, Mika A. Sillanpää, and Pertti J. Hakonen. Operation of a phase qubit as a microwave switch. arXiv, 1103.2631, March 2011.
  • [Publication 9]: Jian Li and G. S. Paraoanu. Dynamical decoupling of superconducting qubits. Journal of Physics: Conference Series, 338, 012010, February 2012.
  • [Publication 10]: Jian Li, M. P. Silveri, K. S. Kumar, J.-M. Pirkkalainen, A. Vepsäläinen, W. C. Chien, J. Tuorila, M. A. Sillanpää, P. J. Hakonen, E. V. Thuneberg, and G. S. Paraoanu. Motional Averaging in a Superconducting Qubit. arXiv, 1205.0675, May 2012.