Cryogenic Thermometry
Measuring temperature at millikelvin requires specialized sensors, since ordinary thermometers freeze out or lose sensitivity in the cold.
Measuring the coldest stages
Knowing the temperature of each stage of a cryostat is essential for operation and diagnosis, but ordinary thermometers do not work at millikelvin temperatures. Specialized sensors are used, chosen for the temperature range they cover, their self-heating, and their sensitivity to magnetic fields, which matters near superconducting magnets and qubits.
Resistance thermometers
The most common cryogenic thermometers are resistors whose resistance changes strongly with temperature. Ruthenium-oxide and carbon-based resistors increase resistance sharply as they cool and remain useful down to tens of millikelvin. They are read with tiny excitation currents to avoid self-heating, because dumping even nanowatts into a cold sensor raises its own temperature and corrupts the reading. Their calibration can shift in magnetic fields, so field-insensitive types are preferred near magnets.
Primary thermometers
Resistance thermometers are secondary: they must be calibrated against a reference. Primary thermometers measure temperature from first principles. Noise thermometry uses the Johnson-Nyquist voltage noise of a resistor, whose power spectral density is proportional to temperature, to read temperature directly. Nuclear-orientation thermometry and Coulomb-blockade thermometry provide other primary references. These are used to calibrate the more convenient secondary sensors.
Thermalization matters
A thermometer only reports the temperature of its own sensing element. If the sensor is poorly heat-sunk, it reads warmer than the stage it is meant to monitor, exactly the thermal-contact problem that afflicts qubits. Careful mounting, heat-sinking of leads, and filtering of the measurement wiring are as important as the sensor itself for an accurate reading.
- Ordinary thermometers fail at millikelvin
- Ruthenium-oxide and carbon resistors are common secondary sensors
- Read with tiny currents to avoid self-heating
- Noise thermometry provides a primary reference