Highly stable biexciton-exciton single photon pairs from CdS/CdSe/CdS quantum shells

Quantum information processing demands efficient Quantum Light Sources (QLS) capable of producing high-fidelity single photons or entangled photon pairs. Colloidal semiconductor QDs offer scalable solution processing and tunable photoluminescence but suffer from broader linewidths and unstable emissions. This leads to spectrally inseparable emission from exciton (X) and biexciton (XX) states, complicating the production of single photons and triggered photon pairs. Here, we demonstrate that colloidal semiconductor Quantum Shells (QSs) achieve significant spectral separation (~ 75-80 meV) and long temporal stability of X and XX emissive states, enabling the observation of exciton-biexciton bunching in colloidal QDs. Our low-temperature single-particle measurements show cascaded XX-X emission of single photon pairs for over 200 seconds, with minimal overlap between X and XX features. The X-XX distinguishability allows for detailed modeling of cross-correlation strength, placing it in perspective with photon pairs of epitaxial QDs. These findings highlight a strong potential of semiconductor quantum shells for applications in quantum information processing.