A High-Sensitivity Superconducting Detector

Abstract

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 (i_c) 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 ω_J=2πV₀/φ₀ and an oscillating voltage across the point contact at ω_J. Upon supplying an rf or microwave current at frequency ω_D, the point contact mixes ω_J and ω_D. Using a homodyne detector at ω_D of bandwidth larger than ω_J, we can retrieve the oscillation at ω_J. Direct measurement of this frequency ω_J yields V₀. Such superconducting circuits may be useful as cryogenic thermometers, as thermal radiation sensors, and in low‐level spectroscopy.