A resistive d.c. SQUID noise thermometer

Abstract

A resistive d.c. SQUID (superconducting quantum interference device), d.c. RSQUID for short, is a superconducting loop that contains two shunted Josephson junctions and a resistor. Our d.c. RSQUIDs are intended for noise thermometry below 4.2 K. They are fully integrated thin film devices fabricated in standard Nb technology. The resistor is made from an evaporated silver layer. It is specially designed in order to suppress the proximity effect, and thus the resistor remains normally conductive down to mK temperatures.

For read-out, modified d.c. SQUID electronics are used. It is directly coupled to the d.c. RSQUID and represents a preamplifier with a voltage noise of 0.8 nV/Hz^1/2, a gain of 4000, and a bandwidth of 4 MHz.

Due to this large bandwidth, the d.c. RSQUID can be operated at a high frequency enabling a short averaging time for the temperature measurement. At a frequency of 2 MHz and a temperature of 4.2 K we measured the temperature with a statistical uncertainty of 0.8% for 500 s averaging time.