  Doppler-free two-photon spectroscopy is a standard technique for precision
measurement of transition frequencies of dipole-forbidden transitions. The
accuracy of such measurements depends critically on fitting of the spectrum to
an appropriate line shape model, often a Voigt profile which neglects the
effect of quantum interference of optical transitions. Here, we report the
observation of quantum interference of optical transition pathways in
Doppler-free two-photon spectroscopy of the cesium 6S-7D transitions. The
quantum interference manifests itself as asymmetric line shapes of the
hyperfine lines of the 7D states, observed through spontaneous emission
following excitation by a narrow-linewidth cw laser. The interference persists
despite the lines being spectrally well-resolved. Ignoring the effect of
quantum interference causes large systematic shifts in the determination of the
line-centers, while accounting for it resolves the apparent line shift and
enables the precise determination of hyperfine splitting in the 7D states. We
calculate the spectral line shape including the effect of quantum interference
and show that it agrees with the experimental observations. Our results have
implications for precision measurements of hyperfine splittings, isotope shifts
and transition frequencies, including those of the hydrogen S-S and S-D
transitions.
