  The quantum Hall effect (QHE) is a cornerstone in the new International
System of Units (SI), wherein the base units are derived from seven fundamental
constants such as Planck's constant h and elementary charge e. Graphene has
revolutionized practical resistance metrology by enabling the realization of
quantized resistance h/2e^2 = 12.9... kOhm under relaxed experimental
conditions. Looking ahead, graphene also has the potential to improve
realizations of the electronic kilogram using the Kibble balance, and the
quantum Ampere in wide current ranges. However, these prospects require
different resistance values than practically achievable in single QHE devices,
while also imposing stringent demands on energy dissipation in single QHE
devices, ultimately requiring currents almost two orders of magnitude higher
than the typical QHE breakdown currents IC ~ 100 uA achievable in graphene.
Here we present unprecedented accuracy in the quantization of a record sized
quantum Hall array (QHA), demonstrating RK/236 ~ 109 Ohm with 0.2
part-per-billion (nOhm/Ohm) accuracy with IC over 5 mA (~ 1 nOhm/Ohm accuracy
for IC = 8.5 mA), using epitaxial graphene on silicon carbide (epigraphene).
The array quantization accuracy, comparable to the most precise universality
tests of QHE in single Hall bar devices, together with the scalability and
reliability of this approach pave the road for superior realizations of three
key units in the modern SI: the ohm, the ampere, and the kilogram.
