  We discuss the effects of multi-phonon excitations on heavy-ion fusion
reactions at energies near and below the Coulomb barrier, focusing especially
on the role of anharmonicities. We carry out a systematic study of those
effects on the excitation function of the fusion cross section and on the
fusion barrier distribution, by using the vibrational limit of the interacting
boson model. We also analyze the recently measured high-precision data of the
$^{16}$O + $^{148}$Sm fusion reaction with this model and discuss the
anharmonic properties of the quadrupole as well as the octupole vibrations in
$^{148}$Sm. Negative and positive static quadrupole moments are deduced for the
first 2$^+$ and 3$^-$ states in $^{148}$Sm, respectively. It is shown that the
fusion barrier distribution strongly depends on the sign of the quadrupole
moments, suggesting that subbarrier fusion reactions offer an alternative
method to extract the static quadrupole moments of phonon states in spherical
nuclei.
