Measurement of sub-picoampere direct currents with uncertainties below ten attoamperes.

A new type of the ultrastable low-noise current amplifier (ULCA) is presented. It involves thick-film resistors to achieve a high feedback resistance of 185 GΩ at the input amplifier. An improved noise level of 0.4 fA/Hz with a 1/f corner of about 30 μHz and an effective input bias current well below 100 aA are demonstrated. For small direct currents, measurement uncertainties below 10 aA are achievable even without current reversal or on/off switching. Above about 1 pA, the stability of the ULCA's resistor network limits the relative measurement uncertainty to about 10 parts per million. The new setup is used to characterize and optimize the noise in the wiring installed on a dilution refrigerator for current measurements on single-electron transport pumps. In a test configuration connected to the wiring in a pulse tube refrigerator, a total noise floor of 0.44 fA/Hz was achieved including the contributions of amplifier and cryogenic wiring.