A light-hole exciton is a quasiparticle formed from a single electron bound to a single light hole. This type of fundamental excitation, if confined inside a semiconductor quantum dot, could be advantageous in quantum information science and technology. However, it has been difficult to access it so far, because confinement and strain in conventional quantum dots favour a ground-state single-particle hole with a predominantly heavy-hole character. Here we demonstrate the creation of a light-hole exciton ground state by applying elastic stress to an initially unstrained quantum dot. Its signature is clearly distinct from that of the well-known heavy-hole exciton and consists of three orthogonally polarized bright optical transitions and a fine-structure splitting of hundreds of microelectronvolts between in-plane and out-of-plane components. This work paves the way for the exploration of the fundamental properties and of the potential relevance of three-dimensionally confined light-hole states in quantum technologies.

Quantum dots (QDs) can act as convenient hosts of two-level quantum szstems, such as single electron spins, hole spins or excitons (bound electron-hole pairs). Due to quantum confinement, the ground state of a single hole confined in a QD usually has dominant heavy-hole (HH) character. For this reason light-hole (LH) states have been largely neglected, despite the fact that may enable the realilzation of coherent photon-to-spin converters or allow for faster spin manipulation compared to HH states. In this work, we use tensile strains larger than 0.3 high quality GaAs/AlGaAs QDs from the conventional HH- to LH-type. The LH-exciton fine structure is characterized by two in-plane-polarized lines and, 400 micro-eV above them, by an additional line with pronounced out-of-plane oscillator strength, consistent with theoretical predictions based on atomistic empirical pseudopotential calculations and a simple mesoscopic model.