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Josephson-point-contact detector radiometer with a pilot signal

В. А. Ильин, A. A. Naumov, V. S. Étkin

Published Feb 1, 1983• 0 citations
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Abstract

We study characteristics of a detector radiometer with a pilot-signal arrangement. This radiometer makes use of the fact that the low-frequency response of a superconducting Josephson point contact to a modulated noisy pilot signal depends on the amplitude of the input signal. The measured fluctuations sensitivity is three times as good as that of an ordinary modulation radiometer using the same Josephson contact.(AIP)

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•Nov 1, 1980•0 citations
We study characteristics of a detector radiometer with a pilot-signal arrangement. This radiometer makes use of the fact that the low-frequency response of a superconducting Josephson point contact to a modulated noisy pilot signal depends on the amplitude of the input signal. The measured fluctuations sensitivity is three times as good as that of an ordinary modulation radiometer using the same Josephson contact.(AIP)
Weakly connected superconductors can function as very low power sensors by utilizing the coherence properties of superconductors. One configuration which has produced significant success incorporates a superconducting point contact and a small resistance element in a low-inductance superconducting circuit. When the critical supercurrent (ic) of the point contact is of the order of φ0/L, (φ0=h/2e, L=inductance of the ring), that circuit exhibits coherent quantum behavior. Used as a parametric amplifier, it has been used to measure ultralow voltages, limited by the thermal noise fluctuations in the resistance element R. The voltage sensitivity is approximated by 8kTR/φ0, where k is Boltzmann's constant and T the absolute temperature of the resistance R. We have measured 4×10−16 V across 1.7×10−10 Ω at 4.2°K with a signal-to-noise ratio approaching 10. The operation of this sensor follows from the Josephson oscillation of voltage-biased superconducting point contacts. A dc voltage V0 across the resistance of the circuit produces an oscillating current at the angular frequency ωJ=2πV0/φ0 and an oscillating voltage across the point contact at ωJ. Upon supplying an rf or microwave current at frequency ωD, the point contact mixes ωJ and ωD. Using a homodyne detector at ωD of bandwidth larger than ωJ, we can retrieve the oscillation at ωJ. Direct measurement of this frequency ωJ yields V0. Such superconducting circuits may be useful as cryogenic thermometers, as thermal radiation sensors, and in low-level spectroscopy.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
The operation of a constant-current quantum interferometer with two Josephson junctions is studied in a system for measuring small changes in a magnetic field with low-frequency modulation of the magnetic flux. Idealized theoretical signal characteristics are found together with their dependence on the operating mode and parameters of a SQUID. These characteristics are used in Pt. II (ref. 12) to determine the minimal detectable change in the magnetic flux, which characterizes the maximal sensitivity of a SQUID.