We report measurements of ultralong coherence in self-assembled quantum dot
s. Transient four-wave mixing experiments at 5 K show an average dephasing
time of 372 ps, corresponding to a homogeneous linewidth of 3.5 mu eV, whic
h is significantly smaller than the linewidth observed in single-dot lumine
scence. Time-resolved luminescence measurements show a lifetime of the dot
ground state of 800 ps, demonstrating the presence of pure dephasing at fin
ite temperature. The homogeneous width is lifetime limited only at temperat
ures approaching 0 K.