Fig. 2: Transmission spectra.

Pump frequency ω_p from the vector network analyzer versus the persistent current energy ε₁ of qubit 1. a Raw data of the observed single-tone spectrum of the sample shown in Fig. 1. b Observed single-tone spectrum with fitted curves corresponding to the state transition frequencies ω_ij between the i-th and j-th eigenstates of Hamiltonian (6). The fit parameters are g₁/2π = 3.33, g₂/2π = 3.45, Δ₁/2π = 1.31, Δ₂/2π = 1.27, ω_r/2π = 5.15, and ε₂/2π = −3.22 GHz. At around ω_p/2π = 5.09 GHz and 5.57 GHz, parasitic modes can be seen, which originate from, for example, sample ground planes and/or the measurement environment, which includes the sample holder and microwave components coupled to the system.