We have performed a metrological characterization of the quantum Hall resistance in a 1 µm wide graphene Hall-bar. The longitudinal resistivity in the center of the ν = ±2 quantum Hall plateaus vanishes within the measurement noise of 20 mΩ upto 2 µA. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5 µA current) equal to that in conventional semiconductors. The principal limitation of the present experiments are the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied. The Hall resistance in two-dimensional electron systems (2DESs) is quantized in terms of natural constants only, R H = h/ie 2 with i an integer number [1]. Due to its high accuracy and reproducibility this quantized Hall resistance in conventional 2DESs is nowadays used as a universal resistance standard [2].Recently a new type of half-integer quantum Hall effect [3,4] was found in graphene, the purely twodimensional form of carbon [5]. Its unique electronic properties [6] (mimicking the behavior of charged chiral Dirac fermions [7,8]) allow the observation of a quantized Hall resistance up to room-temperature [9, 10], making graphene a promising candidate for a high-temperature quantum resistance standard. Although the quantized resistance in graphene around the ν = 2 plateau is generally believed to be equal to h/2e 2 , up to now it has not been shown to meet a metrological standard. In this Letter we present results of the first metrological characterization of the quantum Hall resistance in graphene. In particular, we will address the present accuracy of quantization (15 ppm) and the experimental conditions limiting this accuracy.Our sample consists of a graphene Hall-bar on a Si/SiO 2 substrate forming a charge-tunable ambipolar field-effect transistor (A-FET), where the carrier concentration can be tuned with a back-gate voltage V g [11]. In order to remove most of the surface dopants that make graphene generally strongly hole doped and limit its mobility, we have annealed the sample in-situ for several hours at 380 K prior to cooling it down slowly (∆T /∆t < 3 K/min) to the base temperature (0.35 K) of a toploading 3 He-system equipped with a 15 T superconducting magnet. After annealing, the charge neutrality point in the A-FET was situated at 5 V and the sample dis- * Electronic address: J.Giesbers@science.ru.nl † Electronic address: U.Zeitler@science.ru.nl 2 µm

We present measurements on a pulse-driven ac Josephson voltage standard (ACJVS) for frequencies up to 1 MHz. By automating our setup we were able to perform a systematic study of the frequency dependence of the ACJVS output. We show that the error caused by the voltage leads is the major error source when measuring at frequencies up to 1 MHz. The squared frequency dependence of the error, which is caused by a resonance in the circuit, allows us to make a correction that results in an uncertainty on the µV V−1 level for frequencies up to at least 20 kHz.

A design of an automated cryogenic current comparator (CCC) resistance ratio bridge for routine measurements in a national metrology institute or standards laboratory is described. It employs a type II CCC for use in a low loss liquid helium storage vessel and can be continuously operated from the mains power via specialized isolated supplies. All parameters including the servo control loops are digitally controlled. Noise sources in the system are analysed using the Allan deviation and it is demonstrated that non-white noise sources can be eliminated by choosing an appropriate current reversal rate.

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Over the last 15 years, research in ac Josephson voltage metrology has focused on two fundamentally different systems: the programmable and the pulse-driven Josephson voltage standards (JVSs). This paper reports the first high precision comparison between the two types of JVS. The METAS programmable voltage standard was moved from Switzerland to the Netherlands to be compared with the Dutch pulse-driven system during four days in November 2010. After a careful investigation of the systematic sources of errors, the comparison was made at a frequency of 500 Hz and an rms amplitude of 104 mV. At that level, the voltage difference measured between the fundamental frequency components of the two standards was −0.18 ± 0.26 µV V−1 (k = 2), showing an excellent agreement between the two systems.