Christian Dickel

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.
arxiv pdf

Observation and stabilization of photonic Fock states in a hot radio-frequency resonator

  1. Mario F. Gely,
  2. Marios Kounalakis,
  3. Christian Dickel,
  4. Jacob Dalle,
  5. Rémy Vatré,
  6. Brian Baker,
  7. Mark D. Jenkins,
  8. and Gary A. Steele
Detecting weak radio-frequency electromagnetic fields plays a crucial role in wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum mechanics,
the ultimate limit of a weak field is a single-photon. Detecting and manipulating single-photons at megahertz frequencies presents a challenge as, even at cryogenic temperatures, thermal fluctuations are significant. Here, we use a gigahertz superconducting qubit to directly observe the quantization of a megahertz radio-frequency electromagnetic field. Using the qubit, we achieve quantum control over thermal photons, cooling to the ground-state and stabilizing photonic Fock states. Releasing the resonator from our control, we directly observe its re-thermalization dynamics with the bath with nanosecond resolution. Extending circuit quantum electrodynamics to a new regime, we enable the exploration of thermodynamics at the quantum scale and allow interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.
arxiv pdf