Graphene quantum Hall resistance standard for realizing the unit of electrical resistance under relaxed experimental conditions

Quantum Hall resistance (QHR) standards play an essential role in the traceability route for the units ohm, farad, ampere, and kilogram in the revised International System of Units (SI). While the primary realization of the unit ohm is still mainly based on GaAs QHR standards in extreme conditions, epitaxial graphene on SiC is a promising alternative to develop QHR standards operating under relaxed conditions due to its wide Landau level splitting and strong Fermi level pinning. Here, we show that epitaxial graphene QHR standards can be operated to reach a high accuracy of less than 2 nω/ω (two parts per billion) at a moderate magnetic flux density of B = 4.5 T, high current of I = 232.5 μA, and easier to access temperature of T = 4.2 K, simultaneously. Repeated measurements in these relaxed conditions over 2.5 years demonstrate the temporal stability and robustness of the QHR standard with an accuracy of 2nω/ω. Furthermore, the accuracy of the graphene QHR standards has been maintained without any indications of degradation, even after experiencing long-distance transport of more than 800 km between two metrology institutes. When implemented, graphene QHR standards may lead to broader dissemination of primary resistance standards beyond national metrology institutes, extending to calibration laboratories and industry on-site.