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Bulletin of the American Physical Society

dc SQUID Phase Qubit with Sub-Micron Junction and Interdigitated Capacitor

Anthony J. Przybysz, Hyeokshin Kwon, E. Crowe, Benjamin K. Cooper, Rangga P. Budoyo, Kaushik Mitra, J. R. Anderson, Christopher J. Lobb, Alex J. Dragt, F. C. Wellstood, Sergey P. Gladchenko, V. Zaretsky, Z. Kim, B. S. Palmer, Kevin D. Osborn

Published Mar 18, 2010• 0 citations
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Identifying Sources of Decoherence in a dc SQUID Phase Qubit With a Sub-μm\mu{\rm m}μm Junction and Interdigitated Capacitor
We fabricated a dc SQUID phase qubit with a sub- μm Al/AlOx/Al qubit junction and an interdigitated shunting capacitor on a sapphire substrate. The qubit junction had a critical current of 135 nA, and the isolation junction had a critical current of 8.3 μA. The shunting capacitance was about 1.5 pF. To reduce the unwanted effects of two-level systems and increase the relaxation timeT1, we have removed unnecessary dielectrics, used a small qubit junction area (450 nm × 500 nm), isolated the qubit from the leads with an on-chip LC filter, and fabricated the device on a bare sapphire substrate. However, at a temperature of 20 mK, we foundT1≈ 300 ns and the coherence timeT2≈ 110 ns, which was much lower than one would expect from loss attributed to the leads and to dielectrics in the tunnel junction and substrate. Measurements ofT1versus applied flux (which tuned the qubit frequency) revealed a correlation between the strength of the coupling of the microwave excitation line to the qubit and the rate of energy dissipation in the qubit. This result suggests that the relaxation time was being limited by coupling to the microwave line.
We have fabricated and tested an Al/AlOx/Al dc SQUID phase qubit on a sapphire substrate with a qubit junction area 0.17 μm2and 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 μm2and 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 SiNxcapacitors. From detuned Rabi oscillation measurements, we estimate a relaxation time T1≈ 400 ns and dephasing time TΦ1700 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.