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 μm
²
and critical current I
₀
,
_J
1
= 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 μm
²
and critical current I
₀
,
_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 T
₁
≈ 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.