  We present a technique for increasing the internal quality factor of kinetic
inductance detectors (KIDs) by nulling ambient magnetic fields with a properly
applied magnetic field. The KIDs used in this study are made from thin-film
aluminum, they are mounted inside a light-tight package made from bulk
aluminum, and they are operated near $150 \, \mathrm{mK}$. Since the thin-film
aluminum has a slightly elevated critical temperature ($T_\mathrm{c} = 1.4 \,
\mathrm{K}$), it therefore transitions before the package ($T_\mathrm{c} = 1.2
\, \mathrm{K}$), which also serves as a magnetic shield. On cooldown, ambient
magnetic fields as small as approximately $30 \, \mathrm{\mu T}$ can produce
vortices in the thin-film aluminum as it transitions because the bulk aluminum
package has not yet transitioned and therefore is not yet shielding. These
vortices become trapped inside the aluminum package below $1.2 \, \mathrm{K}$
and ultimately produce low internal quality factors in the thin-film
superconducting resonators. We show that by controlling the strength of the
magnetic field present when the thin film transitions, we can control the
internal quality factor of the resonators. We also compare the noise
performance with and without vortices present, and find no evidence for excess
noise beyond the increase in amplifier noise, which is expected with increasing
loss.
