A team has developed graphene-based quantum Hall resistance standards that use the wye-delta transformation to achieve high accuracy. The technique combines arrays of graphene devices with a connection topology that derives equivalent resistance values from star and triangle configurations, thereby extending the range of accessible values without losing traceability to the quantum Hall effect.

The quantum Hall effect has provided the practical definition of the ohm for decades through the von Klitzing constant, R_K = h/e² ≈ 25 812.807 Ω. Conventional standards, however, are limited to discrete values set by the number of resistors in series or in parallel. The wye-delta approach allows adjustable resistance combinations to be built from identical elements, something particularly useful when the active material is graphene, whose mobility and homogeneity improve the reproducibility of the Hall plateaus.

The wye-delta transformation is a classical circuit-analysis tool: three resistors connected in a star can be replaced by an equivalent triangle (and vice versa) while preserving the impedance between nodes. Applied to an array of graphene quantum Hall devices, this equivalence makes it possible to synthesise resistance values intermediate between the quantised steps, with uncertainties compatible with the demands of electrical metrology.

The result is a high-accuracy resistance standard that widens the range of values available for instrument calibration without resorting to conventional resistors with larger uncertainties. Traceability to the quantum Hall effect is preserved, making it attractive for metrology institutes and for the precision electronics industry. The next step will be to validate the standard in interlaboratory comparisons and to assess its behaviour at different temperatures and magnetic fields.