Practical noise thermometers for low temperatures

Thermometry over broader temperature ranges below 4 K usually requires the application of different thermometers to have sufficient precision and accuracy. We have developed dc SQUID-based noise thermometers which cover the temperature range from 4 K to below 10 mK. The working principle is based on the detection of thermal magnetic flux noise generated by noise currents in a metallic temperature sensor. The noise thermometers exhibit a linear characteristic via the Nyquist relation, are fast, compact, and easy to use. The current sensing noise thermometer and the magnetic field fluctuation thermometer we present can be designed to meet different requirements concerning speed and accuracy of the temperature measurement. In both variants the temperature sensor and the SQUID are located at the same temperature level. Experimental results will be given for the characteristics of the thermometers as well as for comparison measurements with the PLTS-2000. A commercial noise thermometer version now is available through the company MAGNICON.