Single-walled carbon nanotubes (SWNTs) are typically long (greater than or similar to 100 nm) and have been well established as novel quasi one-dimensional systems with interesting electrical, mechanical, and optical properties. Here, quasi zero-dimensional SWNTs with finite lengths down to the molecular scale (7.5 nm in average) were obtained by length separation using a density gradient ultracentrifugation method. Different sedimentation rates of nanotubes with different lengths in a density gradient were taken advantage of to sort SWNTs according to length. Optical experiments on the SWNT fractions revealed that the UV-vis-NIR absorption and photoluminescence peaks of the ultrashort SWNTs blue-shift up to similar to 30 meV compared to long nanotubes, owing to quantum confinement effects along the length of ultrashort SWNTs. These nanotube capsules essentially correspond to SWNT quantum dots.