Microwave amplification via interfering multi-photon processes in a half-waveguide quantum electrodynamics system

  1. Fahad Aziz,
  2. Kuan-Ting Lin,
  3. Ping-Yi Wen,
  4. Samina,
  5. Yu Chen Lin,
  6. Emely Wiegand,
  7. Ching-Ping Lee,
  8. Yu-Ting Cheng,
  9. Ching-Yeh Chen,
  10. Chin-Hsun Chien,
  11. Kai-Min Hsieh,
  12. Yu-Huan Huang,
  13. Ian Hou,
  14. Jeng-Chung Chen,
  15. Yen-Hsiang Lin,
  16. Anton Frisk Kockum,
  17. Guin-Dar Lin,
  18. and Io-Chun Hoi
We investigate the amplification of a microwave probe signal by a superconducting artificial atom, a transmon, strongly coupled to the end of a one-dimensional semi-infinite transmission
line. The end of the transmission line acts as a mirror for microwave fields. Due to the weak anharmonicity of the artificial atom, a strong pump field creates multi-photon excitations among the dressed states. Transitions between these dressed states, Rabi sidebands, give rise to either amplification or attenuation of the weak probe. We obtain a maximum amplitude amplification of about 18 %, higher than in any previous experiment with a single artificial atom, due to constructive interference between Rabi sidebands. We also characterize the noise properties of the system by measuring the spectrum of spontaneous emission.

Scalable collective Lamb shift of a 1D superconducting qubit array in front of a mirror

  1. Kuan-Ting Lin,
  2. Ting Hsu,
  3. Chen-Yu Lee,
  4. Io-Chun Hoi,
  5. and Guin-Dar Lin
We theoretically investigate resonant dipole-dipole interaction (RDDI) between artificial atoms in a 1D geometry, implemented by N transmon qubits coupled through a transmission line.
Similarly to the atomic cases, RDDI comes from exchange of virtual photons of the unexcited modes, and causes the so-called collective Lamb shift (CLS). To probe the shift, we effectively set one end of the transmission line as a mirror, and examine the reflection spectrum of the probe field from the other end. Our calculation shows that when a qubit is placed at the node of the standing wave formed by the incident and reflected waves, even though it is considered to be decoupled from the field, it results in large energy splitting in the spectral profile of a resonant qubit located elsewhere. This directly signals the interplay of virtual photon processes and explicitly demonstrates the CLS. We further derive a master equation to describe the system, which can take into account mismatch of participating qubits and dephasing effects. Our calculation also demonstrates the superradiant and subradiant nature of the atomic states, and how the CLS scales when more qubits are involved.