Errors in a cryogenic current comparator at low flux levels

We present preliminary ratio accuracy tests on a 10000:10000 ratio cryogenic current comparator (CCC) at microamp current levels. We observe an error flux in the CCC’s SQUID detector of ∼ 4 μϕ0 which does not reverse sign when the bridge current source high and low terminals are exchanged. We discuss the possible implications for precision scaling of small currents and electron pump measurements.

I. Introduction

The cryogenic current comparator (CCC) has become a standard piece of instrumentation in metrology laboratories, with its most widespread application being the measurement of resistance ratios. The CCC can establish a precise ratio between two currents I₁ and I₂ by measuring the flux induced by the currents flowing through windings of N₁ and N₂ turns around a superconducting toroid. A superconducting quantum interference device (SQUID) senses the difference in the flux induced by the currents, δϕ = k(N₁I₁ − N₂I₂), where k is the sensitivity of the CCC and associated flux transformers. Using the output of the SQUID as input to a feedback loop controlling one of the current sources then establishes the equality I₁/I₂ = N₂/N₁. This is the basis for resistance ratio bridges performing routine measurements of standard resistors with relative uncertainties at the level of a few parts in 10⁹ [1].