  Measurements of ultrahigh-fidelity absorption spectra can help validate
quantum theory, engineer ultracold chemistry, and remotely sense atmospheres.
Recent achievements in cavity-enhanced spectroscopy using either
frequency-based dispersion or time-based absorption approaches have set new
records for accuracy with uncertainties at the sub-per-mil level. However,
laser scanning5 or susceptibility to nonlinearities limits their ultimate
performance. Here we present cavity buildup dispersion spectroscopy (CBDS) in
which the dispersive frequency shift of a cavity resonance is encoded in the
cavity's transient response to a phase-locked non-resonant laser excitation.
Beating between optical frequencies during buildup exactly localizes detuning
from mode center, and thus enables single-shot dispersion measurements. CBDS
yields an accuracy limited by the chosen frequency standard, a speed limited by
the cavity round-trip time, and is currently 50 times less susceptible to
detection nonlinearity compared to intensity-based methods. The universality of
CBDS shows promise for improving fundamental research into a variety of
light-matter interactions.
