Quantum-precise electrical power

The measurement of electrical a.c. power is of enormous economic importance. Equipment used every day is calibrated in an uninterrupted calibration chain, at the end with the national power standard which is located at PTB. The quantities of current and voltage are thereby recorded with a special sampling voltmeter. In order to calibrate this voltmeter, PTB uses thermal converters which compare the a.c. power with a power at d.c. by means of a thermal effect. The d.c. quantities themselves are ultimately traceable to fundamental constants via the Josephson effect.

Through the development of highly integrated programmable quantum arrays, PTB for the first time succeeded in synthesizing a.c. voltages with an amplitude of 10 V at the frequency of 50 Hz. These extremely exactly reproducible a.c. signals are now used in the electrical power standard. Thereby, the special feature of the sampling procedure – to record current and voltage alternately in time again and again ("sampling") in order to measure the evolution in time of P(t) = U(t) · I(t) and to determine the corresponding power – is exploited: in addition to sampling U and I, a suitably synthesized quantum voltage is sampled in immediate succession. In this way, the voltmeter is calibrated continuously and in real time, replacing the indirect traceability to quantum quantities via the thermal converter by a direct connection to a quantum standard.

The measurement uncertainty of the quantum power standard reaches the same good value of 1 µW/VA as before. This speaks for the very high quality of the classic standard, but also shows that in both methods the limit set by other measuring components has obviously been reached. The significant progress lies in the direct traceability of electrical power to quantum quantities, above all with respect to the re-definition of the system of units, according to which in the future fundamental constants will also be the official basis of electrical units.