Charge qubit driven via the Josephson nonlinearity

  1. Jani Tuorila,
  2. Matti Silveri,
  3. Mika Sillanpää,
  4. Erkki Thuneberg,
  5. Yuriy Makhlin,
  6. and Pertti Hakonen
We study the novel nonlinear phenomena that emerge in a charge qubit due to the interplay between a strong microwave flux drive and a periodic Josephson potential. We first analyze
the system in terms of the linear Landau-Zener-St\“uckelberg model, and show its inadequacy in a periodic system with several Landau-Zener crossings within a drive period. Experimentally, we probe the quasienergy levels of the driven qubit with an LC-cavity, which requires the use of linear response theory. We also show that our numerical calculations are in good agreement with the experimental data.

Dielectric losses in multi-layer Josephson junction qubits

  1. David Gunnarsson,
  2. Juha-Matti Pirkkalainen,
  3. Jian Li,
  4. Gheorghe Sorin Paraoanu,
  5. Pertti Hakonen,
  6. Mika Sillanpää,
  7. and Mika Prunnila
We have measured the excited state lifetimes in Josephson junction phase and transmon qubits, all of which were fabricated with the same scalable multi-layer process. We have compared
the lifetimes of phase qubits before and after removal of the isolating dielectric, SiNx, and find a four-fold improvement of the relaxation time after the removal. Together with the results from the transmon qubit and measurements on coplanar waveguide resonators, these measurements indicate that the lifetimes are limited by losses from the dielectric constituents of the qubits. We have extracted the individual loss contributions from the dielectrics in the tunnel junction barrier, AlOx, the isolating dielectric, SiNx, and the substrate, Si/SiO2, by weighing the total loss with the parts of electric field over the different dielectric materials. Our results agree well and complement the findings from other studies, demonstrating that superconducting qubits can be used as a reliable tool for high-frequency characterization of dielectric materials. We conclude with a discussion of how changes in design and material choice could improve qubit lifetimes up to a factor of four.