Current-voltage (I-V) characteristic of a single, small (100 × 300 nm2), Al-Al2O3-Al Josephson junction, measured at T = 30 mK. The absence of a zero voltage supercurrent branch is due to the small capacitance of the junction (C ≃ 5 fF) completely suppressing the critical current.
I-V curve of an asymmetric dc SQUID, measured at T = 30 mK, with a small Josephson junction as in Fig. 1 . The supercurrent branch of magnitude ∼ 850 nA is evident.
Photograph of asymmetric SQUID before bonding the two halves of the loop together. Junction J1 has an area ∼ 100 × 300 nm2, and is shunted by an additional capacitance C1 ≃ 6 fF to help stabilize its phase, . Junction J2 has an approximate area of ∼ 1.25 × 0.5 μm2.
Circuit schematic of asymmetric SQUID. The small junction J1 in the SQUID is shunted by an on-chip capacitance C1 ≃ 6 fF. The large junction J2 is shunted by its self-capacitance C2.
Switching current versus flux current (I S (IΦ)) data from the SQUID, taken at (a) T = 30 mK and (b) T = 250 mK. The color scale shows the number of switching events recorded out of 104 trials.
Mean SQUID switching current at T = 250 mK (points), with a quadratic fit (red line) to account for device heating from the flux bias current, IΦ. The solid black line is the data corrected for a parabolic offset due to Joule heating.
SQUID design parameters to enable small phase uncertainty inJ1.
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