Reflective Amplification Without Population Inversion from a Strongly Driven Superconducting Qubit
P. Y. Wen1*, A. F. Kockum2, H. Ian3,4, J. C. Chen1, F. Nori2,5, I.-C. Hoi1
1Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
2Center for Emergent Matter Science, RIKEN, Wako, Saitama, Japan
3Institute of Applied Physics and Materials Engineering, University of Macau, Taipa, Macau
4UMacau Zhuhai Research Institute, University of Macau, huhai, Guangdong, China
5Physics Department, The University of Michigan, Ann Arbor, Michigan, USA
* Presenter:P. Y. Wen
 We demonstrate amplification of electromagnetic wave by a strongly driven superconducting qubit at the end of a semi-infinite waveguide without population inversion. When a strong resonant drive field splits the qubit transition into a Mollow triplet, a weak probe field is amplified if it is tuned to the frequencies in-between the resonances in that triplet. We observe a 7% amplitude gain for a weak probe at frequencies in-between the triplet peaks. This amplification is not due to population inversion, neither in the bare qubit basis nor in the dressed-state basis, but instead results from a four-photon process that converts energy from the strong drive to the weak probe. We find excellent agreement between the experimental results and numerical simulations without any free fitting parameters. The device demonstrated here may have applications in quantum information processing and quantum-limited measurements.

Keywords: amplification, superconducting qubit, Mollow triplet, four-photon process