  Advances in ultrafast spectroscopy can provide access to dynamics involving
nontrivial quantum correlations and their evolutions. In coherent 2D
spectroscopy, the oscillatory time dependence of a signal is a signature of
such quantum dynamics. Here we study such beating signals in electronic
coherent 2D spectroscopy of CdSe quantum dots (CdSe QDs) at 77 K. The beating
signals are analyzed in terms of their positive and negative Fourier
components. We conclude that the beatings originate from coherent LO-phonons of
CdSe QDs. No evidence for the quantum dot size dependence of the LO-phonon
frequency was identified.
