Improving the Traceable Measurement and Generation of Small Direct Currents

We present the latest improvements in the traceable measurement and generation of small electric currents. A central tool in our traceability chain for small direct currents is a new binary cryogenic current comparator (CCC) with a total of 18276 turns. This 14-bit CCC is well suited for the calibration of highvalue resistors and current amplifiers, but also for the direct amplification of small currents. A noise level of 5 fA/√Hz at 0.05 Hz is routinely achieved. The systematic uncertainty due to noise rectification was exemplarily investigated in a ratio-error test configuration, showing that a total uncertainty of about one part in 106 can be achieved at 100 pA. For further improvement, a new instrument was developed, the ultrastable low-noise current amplifier (ULCA). Its transfer coefficient is highly stable versus time, temperature, and current amplitude within a full dynamic range of ±5 nA. The ULCA is calibrated with the 14-bit CCC at high current amplitude, and allows the measurement or generation of 100-pA direct current with an uncertainty of one part in 10⁷. The novel setup was successfully used to investigate the uncertainty of the established capacitor charging method. A quantum metrology triangle experiment based on the presented instruments is proposed.