1:30000 cryogenic current comparator with optimum squid readout

We developed a 1:30000 Cryogenic Current Comparator for SET current amplification. A dedicated low-noise, directly-coupled SQUID was used for the readout, which allowed reaching a sensitivity close to ideal. The ratio error was <9/spl times/10/sup -9/. The CCC-SQUID equivalent current input noise was 3.0 fA/Hz/sup 1/2/, measured down to 0.1 Hz.

Introduction

The future establishment of a quantum current standard relies on the very precise amplification of the very small (1–10 pA), quantized current provided by a Single Electron Tunneling device, with the help of an ultra-sensitive, large-ratio Cryogenic Current Comparator [1] with SQUID readout. The current quantum standard could then be combined with the existing resistance and voltage quantum standards to close the quantum metrological triangle of electrical units [2]. The final current resolution of the 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 showed [3] that perfect coupling could be achieved connecting the CCC directly to the SQUID input coil, in which case the optimal current noise could be achieved:

$$\left\langle I_{P}^{2}\right\rangle^{1/2}={1\over N_{CCC}}\sqrt{{8\varepsilon\over k_{\rm sq}^{2}L_{CCC}}},\eqno{\hbox{(1)}}$$ where is the coupling constant between the SQUID washer and input coil. To minimize , the CCC should have a large number of primary turns and maximal self-inductance ; the SQUID should have minimum energy resolution and input coil inductance close to the value of . In this paper we present the integration and characterization of a complete, 1:30 000 CCC with optimal SQUID readout.