FIGURE 4. Quantum beating in cross peaks.

a, The raw amplitude of the exciton 1–3 cross-peak, with a Fourier interpolation of the points (dotted line). b, The power spectrum of this interpolation (dotted line), the exciton beating line spectra of both excitons 1 and 3 (black), and the 1–3 beat frequency (red). We expect that the other frequencies may couple to this cross-peak but that the dominant frequency corresponds to the red transition. The apparent low-frequency peak is due to the growth of the cross-peak amplitude and appears as a peak because the data were de-meaned (mean subtracted from the data) before the transform to improve numerical accuracy, pinning the zero-frequency component to zero.