Colloidal semiconductor nanocrystals (NCs) have become a key material for next-generation electro-optical and quantum technologies. Their versatility in synthesis offers a wide range in the design of e.g. size, shape and composition. Among them, halide-perovskite NCs stand out as exceptionally bright emitters with narrow linewidths and robust photostability at cryogenic temperatures [1]. These properties render them highly attractive for single-photon generation [2, 3]. However, spectral diffusion (SD) – stochastic spectral fluctuations – critically limits the performance of perovskite NCs in quantum-optical applications.

Spectral fluctuations occur over an extremely broad temporal range, from nanoseconds to seconds, far beyond the capabilities of standard photoluminescence-spectroscopy methods that cannot simultaneously provide high time and energy resolution. In II–VI nanoparticles, such as CdSe/CdS core/shell NCs, it has been well established that spectral variations originate from local electric-field fluctuations and can be attributed to the quantum-confined Stark effect [4]. Whether a similar mechanism underlies the SD dynamics in halide-perovskite (e.g. CsPbBr_₃) NCs remains an open and highly relevant question.

To address this question, we use heralded spectroscopy, monitoring the correlations of photon energies as a function of the time between detections. This allows us to observe SD across the entire accessible time range from a single measurement. To achieve high time and energy resolutions, we replace the standard relatively slow camera at the output of a spectrometer with a linear single-photon avalanche diode (SPAD) array. Each of the 64 SPADs independently provides a sub-nanosecond temporal resolution and sub-meV spectral precision [5].

We statistically analyze the difference (δE) and average (⟨E⟩) of the energies for pairs of photons vs. the time difference (τ) between their detections. By performing this evaluation for an exponentially increasing τ we access the dynamics of SD from nanoseconds to seconds. For each time step, we quantify the distribution of δE and ⟨E⟩ across the entire measurement time of a single CsPbBr_₃ NC (a few minutes) at a temperature of 6 K.

Considering a fluctuating source, photons emitted shortly one after the other present a higher energy correlation than two photons separated by a longer time difference. As a result, the distribution of δE for short τ is significantly narrower than that of ⟨E⟩. Our measurements find a power law increase of δE with τ, consistent with a diffusive process, over the entire observed time axis and up to ~10 seconds. Interestingly, we find that both the magnitude and timescale of SD are critically sensitive to the laser excitation power. Importantly, at lower excitation powers, nearly no difference is observed between the distributions of δE with increasing τ, indicating that hardly any spectral fluctuations occur.

Our heralded spectroscopy measurements reveal, for the first time, the power-law progression of spectral fluctuations over 11 orders of magnitude in the time domain. To uncover the microscopic origin of these dynamics, we plan a systematic study across CsPbBr_₃ nanoparticles of varying dimensions and capping ligands. In order to test the influence of the quantum-confined Stark effect, we intend to perform heralded spectroscopy under external electric bias. Such insights will bring us closer to understanding and ultimately suppressing SD in perovskite quantum emitters.