Direct comparison of fractional and integer quantized Hall resistances: Status and perspectives

The direct comparison of fractional and integer quantized Hall resistances with metrological accuracy is of fundamental interest. With respect to the measurement instrumentation, this is a demanding experiment requiring the use of a cryogenic current comparator at very low current levels. We discuss settings and results of a comparison between the resistances for filling factors of 1/3 or of 2, respectively, and consider further improvements.