  Rydberg excitons are, with their ultrastrong mutual interactions, giant
optical nonlinearities, and very high sensitivity to external fields, promising
for applications in quantum sensing and nonlinear optics at the single-photon
level. To design quantum applications it is necessary to know how Rydberg
excitons and other excited states relax to lower-lying exciton states. Here, we
present photoluminescence excitation spectroscopy as a method to probe
transition probabilities from various excitonic states in cuprous oxide, and we
show giant Rydberg excitons at $T=38$ mK with principal quantum numbers up to
$n=30$, corresponding to a calculated diameter of 3 $\mu$m.
