M. J. Schwarz

Tunable coupling of transmission-line microwave resonators mediated by an rf SQUID

  1. F. Wulschner,
  2. J. Goetz,
  3. F. R. Koessel,
  4. E. Hoffmann,
  5. A. Baust,
  6. P. Eder,
  7. M. Fischer,
  8. M. Haeberlein,
  9. M. J. Schwarz,
  10. M. Pernpeintner,
  11. E. Xie,
  12. L. Zhong,
  13. C. W. Zollitsch,
  14. B. Peropadre,
  15. J.J. García-Ripoll,
  16. E. Solano,
  17. K. Fedorov,
  18. E. P. Menzel,
  19. F. Deppe,
  20. A. Marx,
  21. and R. Gross
We realize tunable coupling between two superconducting transmission line resonators. The coupling is mediated by a non-hysteretic rf SQUID acting as a flux-tunable mutual inductance
between the resonators. From the mode distance observed in spectroscopy experiments, we derive a coupling strength ranging between -320MHz and 37 MHz. In the case where the coupling strength is about zero, the microwave power cross transmission between the two resonators can be reduced by almost four orders of magnitude compared to the case where the coupling is switched on. In addition, we observe parametric amplification by applying a suitable additional drive tone.
arxiv pdf

Ultrastrong coupling in two-resonator circuit QED

  1. A. Baust,
  2. E. Hoffmann,
  3. M. Haeberlein,
  4. M. J. Schwarz,
  5. P. Eder,
  6. J. Goetz,
  7. F. Wulschner,
  8. E. Xie,
  9. L. Zhong,
  10. F. Quijandria,
  11. D. Zueco,
  12. J.J. García-Ripoll,
  13. L. Garcia-Alvarez,
  14. G. Romero,
  15. E. Solano,
  16. K. G. Fedorov,
  17. E. P. Menzel,
  18. F. Deppe,
  19. A. Marx,
  20. and R. Gross
We report on ultrastrong coupling between a superconducting flux qubit and a resonant mode of a system comprised of two superconducting coplanar stripline resonators coupled galvanically
to the qubit. With a coupling strength as high as 17% of the mode frequency, exceeding that of previous circuit quantum electrodynamics experiments, we observe a pronounced Bloch-Siegert shift. The spectroscopic response of our multimode system reveals a clear breakdown of the Jaynes-Cummings model. In contrast to earlier experiments, the high coupling strength is achieved without making use of an additional inductance provided by a Josephson junction.
arxiv pdf

Tunable and Switchable Coupling Between Two Superconducting Resonators

  1. A. Baust,
  2. E. Hoffmann,
  3. M. Haeberlein,
  4. M. J. Schwarz,
  5. P. Eder,
  6. E. P. Menzel,
  7. K. Fedorov,
  8. J. Goetz,
  9. F. Wulschner,
  10. E. Xie,
  11. L. Zhong,
  12. F. Quijandria,
  13. B. Peropadre,
  14. D. Zueco,
  15. J.J. García-Ripoll,
  16. E. Solano,
  17. F. Deppe,
  18. A. Marx,
  19. and R. Gross
We realize a device allowing for tunable and switchable coupling between two superconducting resonators mediated by an artificial atom. For the latter, we utilize a persistent current
flux qubit. We characterize the tunable and switchable coupling in frequency and time domain and find that the coupling between the relevant modes can be varied in a controlled way. Specifically, the coupling can be tuned by adjusting the flux through the qubit loop or by saturating the qubit. Our time domain measurements allow us to find parameter regimes for optimal switch performance with respect to qubit drive power and the dynamic range of the resonator input power
arxiv pdf

Gradiometric flux qubits with tunable gap

  1. M. J. Schwarz,
  2. J. Goetz,
  3. Z. Jiang,
  4. T. Niemczyk,
  5. F. Deppe,
  6. A. Marx,
  7. and R. Gross
For gradiometric three-Josephson-junction flux qubits, we perform a systematic study on the tuning of the minimal transition frequency, the so-called qubit gap. By replacing one of
the qubit’s Josephson junctions by a dc SQUID, the critical current of this SQUID and, in turn, the qubit gap can be tuned in situ by a control flux threading the SQUID loop. We present spectroscopic measurements demonstrating a well-defined controllability of the qubit gap between zero and more than 10 GHz. In the future, this enables one to tune the qubit into and out of resonance with other superconducting quantum circuits, while operating the qubit at its symmetry point with optimal dephasing properties. The experimental data agree very well with model calculations based on the full qubit Hamiltonian. From a numerical fit, we determine the Josephson coupling and the charging energies of the qubit junctions. The derived values agree well with those measured for other junctions fabricated on the same chip. We also demonstrate the biasing of gradiometric flux qubits near the symmetry point by trapping an odd number of flux quanta in the gradiometer loop. In this way, we study the effect of the significant kinetic inductance, thereby obtaining valuable information for the qubit design.
arxiv pdf