R. Aguado

Ultrastrong Coupling and Coherent Dynamics in a Gate-Tunable Transmon Qubit

  1. I. Casal Iglesias,
  2. F. J. Matute-Cañadas,
  3. G. O. Steffensen,
  4. A. Ibabe,
  5. L. Splitthoff,
  6. T. Kanne,
  7. J. Nygard,
  8. V. Rollano,
  9. D. Granados,
  10. A. Gomez,
  11. R. Aguado,
  12. A. Levy Yeyati,
  13. and E. J. H. Lee
Ultrastrong light-matter coupling (USC) gives access to exotic quantum phenomena and promises faster quantum gates, yet coherent time-domain control in this regime remains largely unexplored.
Here, we realize USC in a hybrid system consisting of an InAs nanowire-based gatemon qubit coupled to a superconducting resonator. Spectroscopy reveals an avoided crossing that cannot be captured by the Jaynes-Cummings (JC) model, as well as photon-number-dependent transitions whose energies deviate markedly from the JC ladder expected in the strong coupling regime. Beyond demonstrating USC, we achieve time-resolved coherent control of the qubit and measure coherence times comparable to gatemons operating outside the USC regime. These results establish that hybrid semiconductor-superconductor qubits can retain coherent control in USC and provide a platform for exploring quantum dynamics and device concepts in this regime.
arxiv pdf

Majorana oscillations and parity crossings in semiconductor-nanowire-based transmon qubits

  1. J. Avila,
  2. E. Prada,
  3. P. San-Jose,
  4. and R. Aguado
We show that the microwave (MW) spectra in semiconductor-nanowire-based transmon qubits provide a strong signature of the presence of Majorana bound states in the junction. This occurs
as an external magnetic field tunes the wire into the topological regime and the energy splitting of the emergent Majorana modes oscillates around zero energy owing to spatial overlap in finite-length wires. In particular, we discuss how the zero-energy fermion parity crossings arising from Majorana oscillations result in distinct spectroscopic features. In split-junction geometries, the plasma mode couples to the phase-dispersing subgap levels resulting from Majorana hybridization via a Jaynes-Cummings-like interaction. As a consequence of this interaction, higher order plasma excitations in the junction inherit Majorana properties, including the 4π effect. Our results, based on a fully microscopic description of the junction, suggest that MW spectroscopy of nanowire-based transmon qubits provides an interesting alternative to Majorana detection by transport spectroscopy.
arxiv pdf