A quantum-optics technique first developed to measure the apparent sizes of stars has been repurposed to reveal how tiny materials emit light after being struck by X-rays. In a study published in Nature Photonics, researchers report an X-ray-driven version of Hanbury Brown and Twiss interferometry that can measure two crucial properties of scintillators: how quickly they release light and how many optical photons they produce for each incoming X-ray photon. The method works by examining correlations between photons rather than simply recording the total brightness of a sample. That distinction could make it especially valuable for investigating scintillators so small that conventional optical measurements struggle to characterize them.
Scintillators are materials that convert high-energy radiation into visible or near-visible light. When an X-ray deposits energy in a scintillator, the material’s electrons are pushed into excited states. As those excitations relax, the stored energy is released through the spontaneous emission of optical photons. The efficiency and timing of this process determine how useful a scintillator may be in radiation detectors, medical imaging, security systems and scientific instruments. Two materials can produce similar average brightness while differing substantially in emission speed, photon yield or the way their light is distributed in time. Measuring those properties becomes even more difficult when the active material consists of nanocrystals, quantum dots or microscopic structures containing only a tiny volume of matter.
The new approach exploits a signature known as photon bunching. In ordinary classical intuition, independent photons arriving at a detector would appear randomly distributed in time. Quantum optics, however, allows the arrival times of photons to be statistically correlated. Hanbury Brown and Twiss interferometry measures those correlations through the second-order correlation function, written as (g^{(2)}(\tau)), where (\tau) is the time delay between two detected photons. The function compares the probability of detecting photon pairs separated by that delay with the probability expected for completely uncorrelated light. A value above one indicates an enhanced likelihood of paired arrivals, while a value near one is consistent with uncorrelated detection.
The original Hanbury Brown and Twiss experiment, carried out in the 1950s, used intensity correlations to estimate the angular diameters of stars. The method was initially controversial because it demonstrated that information about a source could be extracted from correlations in light intensity even when the light beams were not being combined in the conventional way associated with optical interferometry. It subsequently became a foundation of quantum optics, where researchers use photon statistics to distinguish thermal, coherent and nonclassical sources. In the new work, the same general logic is applied to light generated inside a scintillator by X-ray excitation, linking the physics of astronomical observation with the behavior of matter under high-energy irradiation.
The key insight is that the timing correlations in scintillation light encode the dynamics of the emission process. If excited states release photons over a characteristic time interval, the distribution of photon arrival pairs changes in a measurable way as the delay (\tau) is varied. By analyzing the shape of (g^{(2)}(\tau)), the researchers can infer the intrinsic emission time of the scintillator rather than relying only on the brightness recorded by a detector. The magnitude of the correlation also carries information about the number of optical photons produced by an individual X-ray event. In this sense, the experiment turns photon statistics into a spectroscopic tool: instead of asking only how much light a material emits, it asks how the emitted photons are temporally organized.
One of the most striking observations was strong bunching in extremely small scintillating structures. In quantum-dot superlattices measuring only a few hundred nanometres, the researchers observed (g^{(2)}(0)>50), meaning that photon pairs detected with essentially zero time separation occurred at a rate more than 50 times the uncorrelated reference level under the reported measurement conditions. Such a large value indicates that the photons generated by individual X-ray excitation events are tightly linked in time. The result is not simply a measure of high brightness; it reflects the concentrated, burst-like nature of scintillation and shows that the correlation technique can still recover meaningful emission information from nanoscale samples.
The researchers tested the method across a broad collection of scintillators, including rare-earth-doped oxide single crystals, undoped oxide crystals and perovskite nanocrystals. Rare-earth dopants are commonly introduced into scintillators because their electronic energy levels can provide efficient pathways for converting deposited radiation energy into visible photons. Perovskite nanocrystals, meanwhile, have attracted intense interest because their optical properties can be tuned through composition, size and structure. By applying the same correlation-based measurement to these different classes of materials, the study benchmarks the technique against systems with substantially different microscopic origins of light emission.
The measurements also revealed that scintillation properties depend on experimental conditions, including temperature and X-ray flux. Temperature can alter the movement of carriers, the probability of nonradiative energy loss and the rates at which excited states decay. X-ray flux can influence how frequently excitations are created and whether available emission pathways become occupied or saturated. Conventional measurements of average light output may show that a material changes under these conditions, but they do not necessarily identify which part of the emission process has changed. Correlation measurements can separate changes in emission timing from changes in photon production, offering a more detailed view of the material’s internal response to radiation.
That ability could be particularly important for modern detectors, which increasingly rely on structured materials with dimensions from micrometres down to the nanoscale. In a large crystal, researchers can often collect enough light to determine a decay curve or estimate photon yield using established techniques. In a nanoscale sample, however, the signal may be weak, transient and difficult to distinguish from instrumental background. Photon-correlation measurements make use of the statistical relationships among detected photons, allowing the experiment to probe emission behavior even when the sample’s overall optical output is small. The study’s success with quantum-dot superlattices suggests that methods developed in quantum optics could become useful tools for screening and optimizing miniature scintillators.
The work also illustrates a broader shift in the way scientists study materials that interact with high-energy radiation. X-rays occupy a very different region of the electromagnetic spectrum from the visible photons ultimately emitted by a scintillator, yet the visible output preserves information about the energy-relaxation processes initiated by the X-ray. By measuring (g^{(2)}(\tau)), researchers can access that information through the statistics of the secondary light. The authors describe the approach as a route toward broader use of quantum-optical techniques for materials with complex optical properties in extreme spectral regimes. If developed further, X-ray-driven Hanbury Brown and Twiss spectroscopy could help scientists compare scintillators more precisely, understand how their emission mechanisms respond to temperature and radiation intensity, and design smaller, faster and more efficient detectors without treating nanoscale materials as miniature versions of their bulk counterparts.
