An ultra-low-noise tunnel junction dc SQUID

The design, fabrication, and noise properties of an ultra-low-noise tunnel junction dc SQUID (superconducting quantum interference device) are presented. The two resistively shunted Pb-alloy tunnel junctions are each 3.6 μm diam. The SQUID has an inductance L of ≈11 pH, a critical current per junction, I0 of ≈100 μA, and a resistance per shunt R of ≈2 Ω. At 2.3 K, eI0R≈kBT, and simple theory predicts that the minimum intrinsic energy sensitivity should be ≈2h. The actual minimum value of the measured high-frequency (70 kHz) flux noise at 4.2 K is φn=1.3×10-7φ0 Hz-1/2. This corresponds to an intrinsic energy sensitivity, φ2n/2L, of 3.2×10-33 J Hz-1 or 5h. Lowering the bath temperature to 1.8 K gave no improvement in performance.