A highly stable tunable cryogenic capacitor

In order to realize a quantum capacitance standard, a highly stable vacuum-gap cryogenic capacitor based on a coaxial design has been developed. A tuning mechanism allows to adjust the room temperature value of the capacitance to obtain a value of 1 pF at 4.2 K. At low temperature, the difference between the measured and the target value is smaller than 30 ppm. This summary describes the main characteristics of this tunable capacitor.