QUROPE Quantum Information Processing and Communication in Europe

The amplification of a microwave signal by a strongly driven solid-state two-level quantum system has been experimentally realized. The artificial two-level system is formed by a persistent current (flux) qubit. The microwave signal is applied to a superconducting resonator, which is coupled to the qubit. For the blue detuned driving the relaxation of the qubit results in an effective population inversion in the dressed state basis of the driven two-level system. Therefore when the Rabi frequency of the driven qubit is tuned into resonance with the resonator, the transmission is expected to be enhanced. This statement has been proved experimentally. Amplification as well as linewidth narrowing has been observed. Excellent agreement between the theoretical predictions, obtained in the frame of the dressed state model, and the experimental results has been demonstrated.

Fig. The normalized transmission amplitude at the center frequency of the fundamental mode of the coplanar waveguide resonator as a function of the driving strength of the 3th harmonic and energy bias of the qubit is shown. An optimal point where the increase of transmission is maximal was found.