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Physica B+C

Quantum mixing close to gap frequencies in superconducting tunnel junctions

Dag Winkler

Published Nov 1, 1984• 0 citations
Publisher
DOI
10.1016/0378-4363(84)90214-6

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Applied Physics Letters•Apr 21, 1986•44 citations
We have tested the conversion efficiency of quasi-particle mixers at frequencies close to the superconducting gap frequency. We investigated arrays of aluminum SIS and SIN tunnel junctions at frequencies around 75 GHz and temperatures down to 0.3 K. This frequency corresponds to about 80% of the gap frequency. The experimental conversion loss was large due to severe impedance mismatches. However, we obtained a good agreement (within 1 dB) to the quantum theory of mixing, indicating that nonequilibrium effects do not play a significant role for these low current density junctions.
The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/Nb superconducting tunnel junctions with an on-chip integrated tuning element are measured and analyzed at 680–750 GHz and at 840 GHz. The lowest receiver noise temperatures are 400 K (double side band) at 720 GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz. We compare data of the pumped I–V curves with the quantum theory of mixing and demonstrate good agreement at frequencies well above the gap frequency.
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.