The authors report a type of photoluminescence upconversion that is directly attributable to the strong quantum confinement of charge carriers in PbS nanocrystals and does not involve the usual mechanism of thermally populated intermediate states. Absorption, emission, and excitation spectroscopy, combined with a simple spectral model, reveals that the upconversion process is consistent with single-photon absorption by an extremely broad single nanocrystal absorption line. This type of upconversion is a result of the time-energy uncertainty principle, indicating unusually rapid dephasing in this system, and stands in direct contrast to the mechanisms proposed for upconversion in other types of nanocrystal.