Entwicklung von supraleitenden Quantenbits zur universellen Analyse von Materialdefekten

• Antragsteller: Dr. Jürgen Lisenfeld
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2015 bis 2021
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 276467699

• Erstellungsjahr: 2020

Zusammenfassung der Projektergebnisse

At the focus of this project are microscopic tunneling defects and their role as a source of decoherence in superconducting quantum bits (qubits). While superconducting qubits have arguably become the most promising contender in the race to realize quantum computers, it became clear that defects in the quantum circuit’s materials are a major source of decoherence. Material defects can give rise to parasitic quantum two-level-systems (TLS) when single or few atoms tunnels between two nearby positions in a disordered material. TLS at resonance with a qubit cause energy relaxation, while TLS at low energies may be thermally switching between their states and generate qubit frequency fluctuations and dephasing. For progress towards practical quantum processors, it is vital to understand the microscopic origins of TLS and learn how they can be avoided in circuit fabrication. This was the general goal of this project. A specific central goal was to develop a device allowing one to study individual TLS in virtually arbitrary materials. The device is based on a superconducting Transmon qubit that is connected to an additional capacitor containing the sample material as a dielectric. We demonstrated that this approach is capable to measure individual TLS properties such as electric dipole moment sizes, coherence times, and interactions with phonons or other TLS. We also explored alternative methods to study individual TLS with superconducting resonators. Our secondary objective to study the response of TLS to applied electric fields resulted in a valuable breakthrough. It lead to a method capable of identifying in which circuit part each observed TLS is located: whether it is in a tunnel junction barrier, on the qubit electrode’s surface oxide, or at the metal-substrate interface. This allows one to separately asses the contributions of each circuit interface to total decoherence. The technique provides an ideal too to guide fabrication improvements towards more coherent qubits. We also devoted much attention to the study of interactions between TLS and thermally activated TLS. This mechanism gives rise to the temporal fluctuations of qubit decoherence rates and resonance frequencies which reduce qubit operation fidelities and demand frequent re-calibration.

Projektbezogene Publikationen (Auswahl)

• Decoherence spectroscopy with individual TLS, Scientific Reports 6, 23786 (2016)
  J. Lisenfeld, A. Bilmes, S. Matityahu, S. Zanker, M. Marthaler, M. Schechter, G. Schön, A. Shnirman, G. Weiss, and A.V. Ustinov
  
• Electronic Decoherence of Two-Level Systems in a Josephson junction, Phys. Rev. B 96, 064504 (2017)
  A. Bilmes, S. Zanker, A. Heimes, M. Marthaler, G. Schön, G. Weiss, A.V. Ustinov, and J. Lisenfeld
  
• Rabi noise spectroscopy of individual two-level tunneling defects, Phys. Rev. B 95, 241409(R) (2017)
  S. Matityahu, J. Lisenfeld, A. Bilmes, A. Shnirman, G. Weiss, A.V. Ustinov, and M. Schechter
  
• Transmission-line resonators for the study of individual two-level tunneling systems, Appl. Phys. Lett. 111, 112601 (2017)
  J.D. Brehm, A. Bilmes, G. Weiss, A.V. Ustinov, and J. Lisenfeld
  
• Probing individual tunneling fluctuators with coherently controlled tunneling systems, Phys. Rev. B 97, 180505(R) (2018)
  S.M. Meißner, A. Seiler, J. Lisenfeld, A.V. Ustinov, and G. Weiss
  
• Correlating Decoherence in Transmon Qubits: Low frequency noise by single fluctuators, Phys. Rev. Lett. 123, 190502 (2019)
  S. Schlör, J. Lisenfeld, C. Müller, A. Bilmes, A. Schneider, D.P. Pappas, A.V. Ustinov, and M. Weides
  
• Dynamical Decoupling of Quantum Two-Level Systems by Coherent Multiple Landau Zener Transitions, npj Quantum Information 5, 114 (2019)
  S. Matityahu, H. Schmidt, A. Bilmes, A. Shnirman, G. Weiss, A.V. Ustinov, M. Schechter, and J. Lisenfeld
  
• Electric field spectroscopy of material defects in transmon qubits, npj Quantum Information 5, 105 (2019)
  J. Lisenfeld, A. Bilmes, A. Megrant, R. Barends, J. Kelly, P. Klimov, G. Weiss, J.M. Martinis, and A.V. Ustinov
  
• Towards understanding two-level-systems in amorphous solids – Insights from quantum devices, Rep. Prog. Phys. 82, 124501 (2019)
  C. Müller, J.H. Cole, and J. Lisenfeld
  
• Resolving the positions of defects in superconducting quantum bits, Scientific Reports 10, 3090 (2020)
  A. Bilmes, A. Megrant, P. Klimov, G. Weiss, J.M. Martinis, A.V. Ustinov, and J. Lisenfeld