Quantum resistance metrology in graphene

We performed a metrological characterization of the quantum Hall resistance in a 1 mu m wide graphene Hall bar. The longitudinal resistivity in the center of the nu=+/- 2 quantum Hall plateaus vanishes within the measurement noise of 20 m Omega up to 2 mu A. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5 mu A current) equal to that in conventional semiconductors. The principal limitation of the present experiments is 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.