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Japanese Journal of Applied Physics

Effects of Magnetic Field on the Josephson Noise from an SIS Mixer

Masayoshi Kobayashi, Shinji Ohara, Akira Katsuno, Kiyomi Sakai, Yoshikazu Hayashi

Published Aug 1, 1985• 1 citation
Publisher

Abstract

The effects of a magnetic field on the noise due to the Josephson current in a superconductor-insulator-superconductor (SIS) mixer were investigated. The noise was found to be suppressed by an applied magnetic field small enough not to affect the DC characteristics of the mixer adversely. The quantum mixing effect between 70 GHz and 10 GHz radiations was clearly observed at bias voltages of 1∼3 mV, as a result of the noise suppression.

DOI
10.1143/jjap.24.1095

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The effects of a magnetic field on the noise due to the Josephson current in a superconductor-insulator-superconductor (SIS) mixer were investigated. The noise was found to be suppressed by an applied magnetic field small enough not to affect the DC characteristics of the mixer adversely. The quantum mixing effect between 70 GHz and 10 GHz radiations was clearly observed at bias voltages of 1∼3 mV, as a result of the noise suppression.
Terahertz Superconducting SIS (Superconductor-Insulator-Superconductor) mixers demonstrate ultra-high sensitivity close to the quantum limit. However, the DC characteristics of these mixes are vulnerable to interference, which may lead to decreased stability. In this study, the paper conducts a systematic experimental investigation into the impact of alternating magnetic field interference. It is observed that the DC bias voltage noise of superconducting SIS mixer under alternating magnetic field increases with the current derivative and is positively correlated with the coupling coefficient. Further experiments also reveal that measures such as increasing the distance between the bias source port and radiation source or reducing the area of the receiving circuit can decrease the coupling coefficient. The method can effectively suppress the interference caused by alternating magnetic field on DC characteristics of the terahertz superconducting SIS mixer.
Josephson currents have been observed to drain instabilities and decrease the sensitivity of a heterodyne superconductor-insulator-superconductor (SIS) receiver operated at submillimeter wavelengths. A static magnetic field is usually applied on the SIS junction to reduce the magnitude of its critical Josephson currents and enable quasiparticle mixing. We measured the SIS mixer output power at the intermediate frequency (IF) versus the dc bias voltage of an array of two 1 μm2 Nb/Al–Al2O3/Nb junctions. We show evidence of additional, magnetic-field-sensitive noise, peaked at the Shapiro steps voltages, which is due to high-frequency Josephson currents converted to the IF by quasiparticle mixing.
We have investigated the effect of a moderately strong magnetic field (up to 1200 Gauss, 13 quantum fluxes within a single junction) on the heterodyne mixing performance of a 385-500GHz superconductor-insulator-superconductor mixer. Both experimental and numerical results indicate that the mixer conversion gain can be significantly reduced by a magnetic field. Surprisingly, the mixer noise shows very weak dependence. The overall receiver noise increase is merely caused by the intermediate frequency noise contribution amplified by a factor of gain reduction. At the expense of little system noise degradation, a relatively strong magnet field can make the mixer more stable at the bias point corresponding to its noise minimum
Superconductor-insulator-superconductor (SIS) mixers are currently the lowest-noise devices available for mixing in the 26–110 GHz frequency range. However, a major source of noise is anticipated to be the increased effects of Josephson oscillations within the mixer as higher operating frequencies are achieved. Simulations have been done to demonstrate these effects. Results indicate that the commonly used three-port analysis is valid over a wide range of conditions, but that even minimal Josephson oscillations have a deleterious effect on mixer performance. The junction shunt capacitance, which serves to short out the Josephson oscillations, is thus an extremely important parameter. However, the characterization of the mixer in terms of an ‘‘ωRC’’ product appears to be an oversimplification. A more elaborate approach to mixer characterization is suggested.
We report the investigation of the influence of Josephson pair tunnelling on the sensitivity of an SIS mixer near the superconducting gap of niobium. Hot and cold load measurements were carried out from 600 GHz to 700 GHz, without completely supressing the Josephson effect. We have noticed that when measurements were made at bias points near the first Shapiro step, significantly higher values of Y-factor could be obtained with magnetic field strengths that made the Shapiro step sharper, rather then those that supressed the step. This resulted in a significant improvement in noise temperature of the mixer at those bias points. This observation is important for SIS mixers operating at frequencies near or above gap frequency (ωgap) because the warping of the second negative photon step to the positive display side, narrowing down the bias voltage interval in which measurement of the Y-factor at the first photon step could be done.
The characteristics of SIS (Superconductor-Insulator-Superconductor) harmonic mixing were studied in the millimeter and submillimeter regions. IF (Intermediate Frequency) signals originating from the Josephson effect and the photon-assisted tunneling were observed. Each property was investigated by applying a magnetic field. Compared with mixing based on photon-assisted tunneling, mixing by the Josephson effect required low LO (Local Oscillator) power and gave rise to a smaller conversion loss, but more noise.