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Applied Physics Letters

Arrays of superconducting tunnel junctions as low-noise 10-GHz mixers

S. Rudner, T. Claeson

Published May 15, 1979• 49 citations
Publisher

Abstract

Quasiparticle mixing (biasing at the gap voltage) was studied in arrays of small Pb-O-Pb tunnel junctions. Coherent mixing at 9 GHz gave a conversion loss as low as −5.8 dB and a single sideband mixer noise temperature of 10–40 K. The optimal conversion efficiency was obtained where the bias current microwave response was maximal. The arrays of junctions were rugged, and gave a good impedance match to external circuitry.

DOI
10.1063/1.90615

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Applied Physics Letters•May 1, 1980•76 citations
Quasiparticle mixing (biasing at the gap voltage) was studied in arrays of small Pb-O-Pb tunnel junctions. Coherent mixing at 9 GHz gave a conversion loss as low as −5.8 dB and a single sideband mixer noise temperature of 10–40 K. The optimal conversion efficiency was obtained where the bias current microwave response was maximal. The arrays of junctions were rugged, and gave a good impedance match to external circuitry.
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.
We report high-frequency mixing at 75 GHz using superconductor-insulator-superconductor (SIS) tunnel junction arrays of various length and composition. Short (6-junction) arrays, both of Pb (In) and of Pb, and long (36-junction) arrays of Pb are used. The array geometry and antenna coupling can be scaled to 300 GHz with existing technology. Comparing the experiments to each other and to theory provides criteria for SIS mixer design. The measured single side band (SSB) conversion loss is as good as 2.0±0.9 dB. This is marginally the best published conversion result for any resistive mixer without image rejection, and quantitatively agrees with the quantum theory of mixing. We examine Josephson effect noise in this device and give a rough upper frequency limit for single-junction SIS mixers. Conversion maxima are also noted close to the half-gap singularity present in Pb (In) junctions.
We have investigated high-frequency limits of superconducting quasiparticle mixers from experiments on Al/Al-oxide-In/Al (or Ag) tunnel junctions, and from calculations based on Tucker’s quantum theory of mixing (QTM). The small superconducting energy gap of Al enabled us to study mixer performance at around the junction gap frequency, fg =(Δ1+Δ2)/h≊87 GHz, of superconductor-insulator-superconductor (SIS) mixers, and at twice the junction gap frequency for S-I-normal metal (SIN) mixers ( fg =Δ1/h) with available millimeter wave sources. Because of the low transition temperature of Al (Tc ≊1.3 K), a dilution refrigerator was used for providing temperatures below 1 K. The best mixer conversion efficiency was obtained for a low-resistance (∼34 Ω) six-junction array SIS mixer operated at f/fg ≊1 (in this case 73 GHz). The coupled single sideband conversion gain (with no image rejection) was −13.8 dB and the available conversion gain was estimated to −8.4 dB. Complicating nonequilibrium phenomena made it difficult to compare the performance of these relatively high-current-density mixers to the QTM. However, good agreement between calculated and corresponding measured conversion efficiencies and dynamic resistance values was obtained for the low-current-density mixers. Noise due to switching between rf-induced Josephson steps and interference from Josephson mixing could be quenched by a magnetic field. Several high-frequency limits are considered in this paper (such as Josephson switching noise, nonequilibrium phenomena, pair breaking, and energy gap limitations). Our results indicate that submillimeter wave SIS mixer receivers are likely to perform well, at least up to their gap frequencies, i.e., the THz region for large gap superconductors. Our QTM calculations showed that conversion gain larger than unity should be possible up to twice the gap frequency. Above this frequency the conversion loss will increase rapidly.
We report the observation of heterodyne mixing in superconductor-insulator-superconductor tunnel junctions with significant conversion gain and with a noise temperature comparable to the photon noise limit. (Double sideband L−1=1.400.14, TM≲1.5 K at 36 GHz.) The mixing arises from the strong nonlinearity in the quasiparticle tunneling currents at voltages comparable to the full energy gap. Gain is observed when the onset of quasiparticle current is sufficiently sharp that quantum effects are important. The observed gain is in good quantitative agreement with calculations based on the work of Tucker. It should make possible the construction of photon-noise-limited microwave heterodyne receivers.