  Next-generation X-ray observatories, such as the Lynx X-ray Observatory
Mission Concept, will require detectors with high quantum efficiency (QE)
across the soft X-ray band to observe the faint objects that drive their
mission science cases. Hybrid CMOS Detectors (HCDs), a form of active-pixel
sensor, are promising candidates for use on these missions because of their
faster read-out, lower power consumption, and greater radiation hardness than
detectors used in the current generation of X-ray telescopes. In this work, we
present QE measurements of a Teledyne H2RG HCD. These measurements were
performed using a gas-flow proportional counter as a reference detector to
measure the absolute flux incident on the HCD. We find an effective QE of $95.0
\pm 1.1\%$ at the Mn K$\alpha$/K$\beta$ lines (at 5.9 and 6.5 keV), $98.5 \pm
1.8\%$ at the Al K$\alpha$ line (1.5 keV), and $85.0 \pm 2.8\%$ at the O
K$\alpha$ line (0.52 keV).
