We have prepared by chemical beam epitaxy extremely high quality Ga0.47In0.53As/InP quantum wells with thickness as thin as 6 A˚. Emission as short as 1.09 &mgr;m at 2 K (1.14 &mgr;m at 300 K) was obtained. Very sharp intense efficient luminescence peaks due to excitonic transitions were obtained from all quantum wells. The photoluminescence (PL) linewidths at 2 K were the narrowest that have been ever reported for Ga0.47In0.53As quantum wells grown by any technique. In fact, these Ga0.47In0.53As quantum well linewidths are at least equal to the narrowest linewidths ever reported for the perfected GaAs/AlAs and GaAs/AlxGa1−xAs quantum wells. These linewidths indicate the ‘‘effective’’ interface roughness to be 0.12 lattice constant, which can be interpreted as that the quantum well was largely consisting of a big domain of the same thicknessLzperforated with a small fraction of small domains of (Lz+a0/2), wherea0(=5.86 A˚) is the lattice constant. No broadening due to band filling from impurities was found. Alloy broadening in Ga0.47In0.53As was limited to the intrinsic value of 1.3 meV. The PL energy upshifts measured in Ga0.47In0.53As quantum wells were in excellent agreement with theoretical values.