A quantum impedance standard having an insignificant, experimentally verified frequency dependence

Recently we have published a novel double-shield configuration of quantum Hall devices to eliminate the effect of all presently known ac losses within 3×10^-9 kHz^-1. Now we have further verified the double-shielding technique by supporting the experimental findings with a model. This model indicates that it should be possible to reduce the frequency dependence to less than 1×10^-9 kHz^-1, which would be better than our ultimate measuring limit and much better than our calculable impedance artefacts. Thereby, the ac quantum Hall impedance closely approaches a perfect quantum standard.