High quality tantalum superconducting tunnel junctions for microwave mixing in the quantum limit

We report on a novel step-defined process used to produce small area (1–6 μm2), high critical current density (102–104 A/cm2) superconducting tunnel junctions with Ta base electrodes and Pb0.9Bi0.1 counterelectrodes. These junctions have extremely small subgap leakage currents, a ‘‘sharp’’ current rise of width ΔV∼30 μV at the sum-gap voltage, and show strong quantum effects when used as microwave mixers. Accurate measurements at 34.5 GHz with a 1.5 GHz intermediate frequency (IF) give single sideband mixer gains up to G=1.1±0.1 and mixer noise temperatures as low as TM=3.8±1.0 K. This value of TM is close to the quantum limit TQ=ℏω/k ln 2=2.5 K. The lowest receiver noise temperature measured at the 1.3 K input to the mixer block, TR=24±1 K, is the best value reported for any heterodyne receiver. Infinite available gain is obtained for a low (25 MHz) IF and a coupled gain as large as 1.6±0.4 is observed.