Bright Exciton Fine Structure Observed in Single CdSe Nanocrystal Quantum Dots

The fine structure splitting of bright excitons in epitaxial quantum dots provides a basis for many quantum computation and entanglement schemes. We demonstrate the existence of a similar splitting in single colloidal CdSe nanocrystals through high- resolution, polarization-resolved, low-temperature photoluminescence (PL) experiments. At 4K, single-dot spectra reveal emission from two distinct, linearly- (and orthogonally- ) polarized bright exciton states. This splitting of the nominally degenerate spin $\pm1$ bright excitons ranges from 1 to 2 meV, depending on nanocrystal size. These values agree well with the splitting recently inferred from spin-polarized resonant PL of nanocrystal ensembles measured in high magnetic fields to 33 Tesla [1]. Similarly to epitaxially-grown quantum dots, the observed fine structure likely results from shape anisotropy of the nanocrystal (i.e. a reduction of axial symmetry), leading to a long-range, anisotropic electron-hole exchange. [1] M. Furis et al., cond-mat/0511567.