A. Baust

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

Squeezing with a flux-driven Josephson parametric amplifier

  1. L. Zhong,
  2. E. P. Menzel,
  3. R. Di Candia,
  4. P. Eder,
  5. M. Ihmig,
  6. A. Baust,
  7. M. Haeberlein,
  8. E. Hoffmann,
  9. K. Inomata,
  10. T. Yamamoto,
  11. Y. Nakamura,
  12. E. Solano,
  13. F. Deppe,
  14. A. Marx,
  15. and R. Gross
Josephson parametric amplifiers (JPA) are promising devices for applications in circuit quantum electrodynamics (QED) and for studies on propagating quantum microwaves because of their
good noise performance. In this work, we present a systematic characterization of a flux-driven JPA at millikelvin temperatures. In particular, we study in detail its squeezing properties by two different detection techniques. With the homodyne setup, we observe squeezing of vacuum fluctuations by superposing signal and idler bands. For a quantitative analysis we apply dual-path cross-correlation techniques to reconstruct the Wigner functions of various squeezed vacuum and thermal states. At 10 dB signal gain, we find 4.9+-0.2 dB squeezing below vacuum. In addition, we discuss the physics behind squeezed coherent microwave fields. Finally, we analyze the JPA noise temperature in the degenerate mode and find a value smaller than the standard quantum limit for phase-insensitive amplifiers.
arxiv pdf

Path Entanglement of Continuous-Variable Quantum Microwaves

  1. E. P. Menzel,
  2. R. Di Candia,
  3. F. Deppe,
  4. P. Eder,
  5. L. Zhong,
  6. M. Ihmig,
  7. M. Haeberlein,
  8. A. Baust,
  9. E. Hoffmann,
  10. D. Ballester,
  11. K. Inomata,
  12. T. Yamamoto,
  13. Y. Nakamura,
  14. E. Solano,
  15. A. Marx,
  16. and R. Gross
Path entanglement constitutes an essential resource in quantum information and communication protocols. Here, we demonstrate frequency-degenerate entanglement between continuous-variable
quantum microwaves propagating along two spatially separated paths. We combine a squeezed and a vacuum state using a microwave beam splitter. Via correlation measurements, we detect and quantify the path entanglement contained in the beam splitter output state. Our experiments open the avenue to quantum teleportation, quantum communication, or quantum radar with continuous variables at microwave frequencies.
arxiv pdf