The development of room-temperature quantum light sources remains a central challenge in quantum technologies. Metal halide perovskite quantum dots (QDs) are promising candidates due to their non-classical emission, including photon anti-bunching and bunching. However, photon bunching — a key signature of collective quantum states — has so far been demonstrated in perovskites only at cryogenic temperatures. In this presentation I will discuss collective blinking and photon bunching at room temperature from individual sub-wavelength CsPbBr3 QD superlattices.

Photoluminescence microscopy measurements showed that over 95% of superlattices exhibit collective two-level blinking, switching between a strongly emitting ON-state and a weak grey state, with ON-state intensity exceeding a single QD by more than two orders of magnitude — indicating synchronized emission from many QDs. Photon coincidence measurements revealed bunching in the ON-state with a degree up to 3.9 across 49 superlattices, in clear contrast to the anti-bunching of single QDs and the absence of bunching in QD ensembles.

Super-resolution imaging localized the ON-state emission to a 20–30 nm region, far smaller than the superlattice itself, pointing to long-range exciton migration toward a localized energy trap. Extended ON-state PL lifetimes (28–69 ns) further support multi-step Förster transfer prior to emission. Power spectral density analysis fits a stretched Lorentzian function, consistent with a well-defined photoactivatable quencher at the confined emission site driving the collective blinking.

Photon bunching is attributed to biexciton–exciton cascade emission at this localized site, where exciton funneling raises local exciton density enough to favor biexciton formation. This is supported by the decrease in bunching degree with increasing excitation power, a hallmark of cascade emission distinct from superfluorescence or superradiance. Fitting the central g2(τ) peak yields a fast ~1.3 ns component attributed to the biexciton lifetime, consistent with the known fast biexciton-to-exciton radiative rate in CsPbBr3. A red-shifted PL shoulder (binding energy 10–29 meV), absent in isolated QDs, further supports biexciton involvement.

These findings establish sub-wavelength CsPbBr3 QD superlattices as a practical platform for room-temperature collective optical phenomena, with promising implications for entangled photon sources and quantum optoelectronic devices.