  By directly coupling a quantum anomalous Hall resistor to a programmable
Josephson voltage standard, we have implemented a quantum current sensor (QCS)
that operates within a single cryostat in zero magnetic field. Using this QCS
we determine values of current within the range 9.33 nA - 252 nA, providing a
realization of the ampere based on fundamental constants and quantum phenomena.
The relative Type A uncertainty is lowest, 2.30 $\times$10$^{-6}$ A/A, at the
highest current studied, 252 nA. The total root-sum-square combined relative
uncertainty ranges from 3.91 $\times$10$^{-6}$ A/A at 252 nA to 41.2
$\times$10$^{-6}$ A/A at 9.33 nA. No DC current standard is available in the
nanoampere range with relative uncertainty comparable to this, so we assess our
QCS accuracy by comparison to a traditional Ohm's law measurement of the same
current source. We find closest agreement (1.46 $\pm$ 4.28)$\times$10$^{-6}$
A/A for currents near 83.9 nA, for which the highest number of measurements
were made.
