Benoit Douçot

Symmetries and model reduction in a cos2φ transmon qubit

  1. Erwan Roverc'h,
  2. Kyrylo Gerashchenko,
  3. Alvise Borgognoni,
  4. Benoit Douçot,
  5. Alexandru Petrescu,
  6. Rémi Robin,
  7. and Zaki Leghtas
Engineered symmetries in superconducting circuits can protect quantum information; much like selection rules in atomic physics, they forbid transitions between the qubit states. Protected
circuits, however, combine several loops and islands and host multiple nonlinearly coupled modes, calling for reduced low-energy models that inherit the circuit symmetries and make the protection manifest. We develop such a reduction for the cos2φ transmon qubit, recently realized experimentally with a two-Cooper-pair interferometer, whose circuit hosts three coupled modes. Exploiting the separation of energy scales between the heavy transmon mode and the light internal modes of the interferometer, we derive an effective single-band Born-Oppenheimer model, including its geometric correction, and extend it to several bands. We identify the symmetries of the full and reduced Hamiltonians and derive exact selection rules. Finally, we identify a parameter regime beyond the single-band description that suppresses flux-noise dephasing without degrading charge-noise protection.
arxiv pdf

Experimental realization of a cos(2φ) transmon qubit

  1. Erwan Roverc'h,
  2. Alvise Borgognoni,
  3. Marius Villiers,
  4. Kyrylo Gerashchenko,
  5. W. Clarke Smith,
  6. Christopher Wilson,
  7. Benoit Douçot,
  8. Alexandru Petrescu,
  9. Philippe Campagne-Ibarcq,
  10. and Zaki Leghtas
Superconducting circuits with embedded symmetries are good candidates to robustly protect quantum information from dominant error channels. The cos(2φ) qubit, consisting of an island
shunted to ground through a tunneling element that selectively transmits pairs of Cooper pairs, leverages charge-parity symmetry to protect from charge-induced errors. In this experiment, we observe a doublet of states of opposite Cooper-pair parity split by 13.6 MHz. Operating in a soft-transmon regime, this splitting is two orders of magnitude smaller than in previous implementations, pushing charge-induced losses well beyond the measured coherence times. Despite the low transition frequency, we demonstrate coherent qubit control, single-shot readout, and resolve quantum jumps. Charge protection of the qubit is evidenced by a 100−fold suppression of the island charge matrix element compared to the unprotected plasmon transition, placing dielectric loss limits above 10 ms. The measured T1=70 μs and Techo2=2.5 μs are instead limited by 1/f flux noise in the tunnelling element’s loop. This experiment shows that pushing Cooper-pair pairing in the transmon regime sets high limits on charge-induced losses while preserving coherent control and single-shot readout of the low-frequency qubit. We identify flux noise as the dominant remaining limitation, calling for gradiometric designs or novel 4e-tunneling elements.
arxiv pdf