Contents I. Introduction
The future establishment of a quantum standard for electrical current relies on the precise amplification of the very small quantized current (1 to 10 pA) provided by a single electron transport (SET) device, with the help of an ultra-sensitive, large-ratio cryogenic current comparator [1] with SQUID readout. The current quantum standard can then be combined with the existing resistance and voltage quantum standards to close the quantum metrological triangle of electrical units [2], [3]. The final current resolution of the cryogenic current comparator (CCC)-SQUID system (provided that all external sources of noise can be properly shielded) depends on the SQUID noise, and the efficiency of the coupling between the CCC and the SQUID. Usually, the CCC overlapped tube is coupled to the SQUID via a superconducting flux transformer. We have shown [4] that perfect coupling can be achieved connecting the tube directly to the SQUID input coil, in which case an optimal current noise is achieved \left\langle {I_{P}^{2}} \right\rangle ^{1/2}={1 \over {N_{\rm CCC}}}\sqrt {{{8\varepsilon } \over {k_{\rm sq}^{2}L_{\rm CCC}}}}.
