DC SQUID Phase Qubit Coupled to an On-Chip LC Resonator

We have fabricated and tested an Al/AlO_x/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area 0.17 μm² and critical current I₀,_J1 = 1.4 μA. The qubit is shunted by an interdigitated capacitor and isolated from the bias leads by an inductive isolation network using a larger Josephson junction with area 2 μm² and critical current I₀,_J2 = 29 μA. Additional high-frequency filtering is provided by an on-chip LC filter, which consists of square spiral inductors and parallel plate SiN_x capacitors. From detuned Rabi oscillation measurements, we estimate a relaxation time T₁ ≈ 400 ns and dephasing time T_Φ 1700 ns. Additionally, we observe that the qubit transition spectrum has multiple equally spaced subpeaks. These subpeaks are caused by coupling between the phase qubit and the LC filter, forming a Jaynes-Cummings system. Each individual subpeak corresponds to transitions with different photon numbers in the LC filter.