  Next-generation X-ray observatories, such as the Lynx X-ray Observatory
Mission Concept or other similar concepts in the coming decade, will require
detectors with high quantum efficiency (QE) across the soft X-ray band to
observe the faint objects that drive their mission science objectives. Hybrid
CMOS Detectors (HCDs), a form of active-pixel sensor, are promising candidates
for use on these missions because of their fast read-out, low power
consumption, and intrinsic radiation hardness. In this work, we present QE
measurements of a Teledyne H2RG HCD, performed using a gas-flow proportional
counter as a reference detector. We find that this detector achieves high QE
across the soft X-ray band, with an effective QE of $94.6 \pm 1.1 \%$ at the Mn
K$\alpha$/K$\beta$ energies (5.90/6.49 keV), $98.3 \pm 1.9 \%$ at the Al
K$\alpha$ energy (1.49 keV), $85.6 \pm 2.8 \%$ at the O K$\alpha$ energy (0.52
keV), and $61.3 \pm 1.1 \%$ at the C K$\alpha$ energy (0.28 keV). These values
are in good agreement with our model, based on the absorption of detector
layers. We find similar results in a more restrictive analysis considering only
high-quality events, with only somewhat reduced QE at lower energies.
