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Applied Physics Letters

Large gain, negative resistance, and oscillations in superconducting quasiparticle heterodyne mixers

William R. McGrath, P. L. Richards, A. D. Smith, Herman van Kempen, R. A. Batchelor, D. E. Prober, Padmanabhan Santhanam

Published Oct 15, 1981• 68 citations
Publisher

Abstract

We have measured the properties of a superconductor-insulator-superconductor quasiparticle mixer which is operated in the quantum limit. Single sideband conversion gain larger than +4 dB was observed at 36 GHz with a mixer noise temperature T = 9±6 K, which is to be compared with the (Planck) quantum limit h/ω/(k ln 2)≃2.5 K. Complete three-port mixer calculations are presented which are in good agreement with the gain measurements. The dynamic resistance was observed to become infinite and then negative as the source conductance was decreased. This implies that arbitrarily large gain is available. The negative resistance is accompanied by IF oscillations.

DOI
10.1063/1.92809

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STIN•Oct 1, 1978•1 citation
We have measured the properties of a superconductor-insulator-superconductor quasiparticle mixer which is operated in the quantum limit. Single sideband conversion gain larger than +4 dB was observed at 36 GHz with a mixer noise temperature T = 9±6 K, which is to be compared with the (Planck) quantum limit h/ω/(k ln 2)≃2.5 K. Complete three-port mixer calculations are presented which are in good agreement with the gain measurements. The dynamic resistance was observed to become infinite and then negative as the source conductance was decreased. This implies that arbitrarily large gain is available. The negative resistance is accompanied by IF oscillations.
We have measured the properties of a superconductor-insulator-superconductor (SIS) quasiparticle mixer which is operated in the quantum limit. Single sideband conversion gain larger than +4 dB was observed at 36 GHz with a mixer noise temperature T=9 + or - 6K, which is to be compared with the (Planck) quantum limit h-bar omega/(kln2) approx. 2.5K. Complete 3-port mixer calculations are presented which are in good agreement with the gain measurements. Negative resistance was observed which implies that arbitrarily large gain is available.
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.
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.
The rapid onset of quasiparitcle tunneling current in superconductor-insulator-superconductor (Josephson) junctions at voltages above 2Δ/e is being used for millimeter-wave heterodyne mixing. Junctions with a 2-μm diameter and RN=50 Ω have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Double sideband conversion efficiencies of 0.32 and mixer noise temperatures as low as TM⩽7 K=4hν/k have been observed. The results are compared with shot-noise-limited mixer theory. Photon-assisted tunneling effects are seen which indicate the approach to photon-noise-limited operation.
The rapid onset of quasiparticle tunneling current in superconductor-insulator-superconductor (Josephson) junctions at voltages above 2 delta/e is being used for millimeter wave heterodyne mixing. Junctions with 2 micron diameter and RN = 50 ohms have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Single sideband conversion efficiencies of 0.18 and mixer noise temperatures as low as TM approximately 14K have been observed. Both results agree with shot noise limited mixer theory. Photon assisted tunneling effects are seen which indicate the approach to photon noise limited operation. (Author)