  We explore orbital dynamics in the spin liquid candidate Ba3CuSb2O9 using
multi-frequency electron spin resonance. We prepared two high quality single
crystals. The crystal with a slight copper deficiency shows a structural phase
transition at around 200 K due to the cooperative Jahn-Teller effect,
accompanied with orbital ordering. In contrast, the crystal with almost perfect
stoichiometry shows no orbital ordering down to the lowest temperature of 1.5
K. Dramatic change in the g-factor anisotropy as a function of frequency and
temperature demonstrates orbital quantum fluctuations at a nearly constant time
scale of ~ 100 ps below 20 K, evidencing the emergence of an orbital liquid
state in this quantum spin liquid compound.
