Observation of Josephson Harmonics in Tunnel Junctions

  1. Dennis Willsch,
  2. Dennis Rieger,
  3. Patrick Winkel,
  4. Madita Willsch,
  5. Christian Dickel,
  6. Jonas Krause,
  7. Yoichi Ando,
  8. Raphaël Lescanne,
  9. Zaki Leghtas,
  10. Nicholas T. Bronn,
  11. Pratiti Deb,
  12. Olivia Lanes,
  13. Zlatko K. Minev,
  14. Benedikt Dennig,
  15. Simon Geisert,
  16. Simon Günzler,
  17. Sören Ihssen,
  18. Patrick Paluch,
  19. Thomas Reisinger,
  20. Roudy Hanna,
  21. Jin Hee Bae,
  22. Peter Schüffelgen,
  23. Detlev Grützmacher,
  24. Luiza Buimaga-Iarinca,
  25. Cristian Morari,
  26. Wolfgang Wernsdorfer,
  27. David P. DiVincenzo,
  28. Kristel Michielsen,
  29. Gianluigi Catelani,
  30. and Ioan M. Pop
An accurate understanding of the Josephson effect is the keystone of quantum information processing with superconducting hardware. Here we show that the celebrated sinφ current-phase
relation (CφR) of Josephson junctions (JJs) fails to fully describe the energy spectra of transmon artificial atoms across various samples and laboratories. While the microscopic theory of JJs contains higher harmonics in the CφR, these have generally been assumed to give insignificant corrections for tunnel JJs, due to the low transparency of the conduction channels. However, this assumption might not be justified given the disordered nature of the commonly used AlOx tunnel barriers. Indeed, a mesoscopic model of tunneling through an inhomogeneous AlOx barrier predicts contributions from higher Josephson harmonics of several %. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The measurement of Josephson harmonics in the CφR of standard tunnel junctions prompts a reevaluation of current models for superconducting hardware and it offers a highly sensitive probe towards optimizing tunnel barrier uniformity.

Parity Detection of Propagating Microwave Fields

  1. Jean-Claude Besse,
  2. Simone Gasparinetti,
  3. Michele C. Collodo,
  4. Theo Walter,
  5. Ants Remm,
  6. Jonas Krause,
  7. Christopher Eichler,
  8. and Andreas Wallraff
The parity of the number of elementary excitations present in a quantum system provides important insights into its physical properties. Parity measurements are used, for example, to
tomographically reconstruct quantum states or to determine if a decay of an excitation has occurred, information which can be used for quantum error correction in computation or communication protocols. Here we demonstrate a versatile parity detector for propagating microwaves, which distinguishes between radiation fields containing an even or odd number n of photons, both in a single-shot measurement and without perturbing the parity of the detected field. We showcase applications of the detector for direct Wigner tomography of propagating microwaves and heralded generation of Schrödinger cat states. This parity detection scheme is applicable over a broad frequency range and may prove useful, for example, for heralded or fault-tolerant quantum communication protocols.