  We propose a new mechanism to produce axion dark matter from inflationary
fluctuations. Quantum fluctuations during inflation are strengthened by a
coupling of the axion kinetic term to the inflaton, which we parametrize as an
effective curvature $\kappa$ in the axion equation of motion. A nonvanishing
curvature breaks the scale invariance of the axion power spectrum, driving a
quantum phase transition with $\kappa$ as the order parameter. The axion power
spectrum is proportional to the inverse comoving horizon to the power of
$\kappa$. For positive $\kappa$ the spectrum gets a red tilt, leading to an
exponential enhancement of the axion abundance as the comoving horizon shrinks
during inflation. This enhancement allows sufficient axion production to
comprise the entire dark matter relic abundance despite the ultralight mass.
Our mechanism predicts a significantly different parameter space from the usual
misalignment mechanism. It allows for axion-like particle dark matter with a
much lower decay constant and thus a larger coupling to Standard Model
particles. Much of the parameter space can be probed by future experiments
including haloscopes, nuclear clocks, CASPEr, and CMB-S4. We can also generate
heavier QCD axion dark matter than the misalignment mechanism.
