ISEC 2013 Special Gordon Donaldson Session: Remembering Gordon Donaldson - 7 of 7 - SQUID-based noise thermometers for sub-Kelvin thermometry
We have developed compact, fast and easy-to-use dc SQUID-based noise thermometers for the temperature range accessible with dilution refrigerators, i.e., from ca. 1K to 1mK. Two implementations have been realized: Magnetic Field Fluctuation Thermometers and Current Sensing Noise Thermometers. In both thermometers, the thermally induced motion of charge carriers in metallic temperature sensors cause thermal magnetic flux noise. SQUID sensors optimized for the detection of this noise are employed, and the sought temperature is extracted from the thermal noise power spectrum using the Nyquist theorem. For the calibration of the thermometers, one reference measurement at a known temperature suffices. We have developed a procedure for reference measurements with direct traceability to the Provisional Low Temperature Scale (PLTS)-2000 as well as methods to determine the uncertainty of the temperature estimates. SQUID sensor design, thermal anchoring and electro-magnetic shielding of our SQUID noise thermometer configurations will be discussed and validation measurements in comparison with the PLTS-2000 will be presented. The results of the validation measurements attest the high linearity, accuracy and speed of our noise thermometers. The devices exhibit noise temperatures below 0.1mK, and estimates of the sought temperature with relative uncertainties of <1% are obtained within measurement times of a few seconds.
SQUID-based noise thermometers for sub-Kelvin thermometry