Observing fast phase transitions in individual nanocrystals through single-photon detection

Philipp Gebauer^a, Frieder Conradt^b^,^c, Ksenia Shcherbak^d, Eva Haage^a, Chenglian Zhu^d, Ihor Cherniukh^d, Claudio Bruschini^e, Alfred Leitenstorfer^a, Maksym Kovalenko^d, Ron Tenne^b^,^c

^aDepartment of Physics, University of Konstanz, 78464 Konstanz, Germany,

^bShulich faculty of chemistry, Technion Israel

^cTechnion - Israel Institute of Technology, the Helen Diller Quantum Center

^dDepartment of Chemistry and Applied Biosciences, Laboratory of Inorganic Chemistry, ETH Zürich, Vladimir-Prelog-Weg 1, Zürich CH-8093, Switzerland

^eAdvanced Quantum Architecture Laboratory (AQUA), EPFL, Neuchatel, Switzerland

Proceedings of MATSUS Spring 2026 Conference (MATSUSSpring26)

H2 Halide perovskites for quantum technologies

Barcelona, Spain, 2026 March 23rd - 27th

Organizers: Quinten Akkerman, Simon Boehme and Maksym Kovalenko

Invited Speaker, Ron Tenne, presentation 726
Publication date: 15th December 2025

Quantum dots (QDs) offer a tunable and technology-compatible platform for quantum light-matter interactions and the inspiring technology that comes along with it. The extent of this promise is directly related to our ability to understand and control this interaction. However, understanding the electronic dynamics is tremendously challenging, in large part due to the large range of time scales in which they occur.

Spectral diffusion (SD), the stochastic fluctuation in the energy of photons emitted from a QD, is a prime example observed on nanoseconds and up to minutes. These significantly erode the potential of QDs to produce indistinguishable photons – a critical resource for quantum technologies – even at low temperatures. In this talk, I will present how heralded spectroscopy, a novel technique based on single-photon detection, can explore SD in unprecedented details.

Relying on a single-photon avalanche detector (SPAD) array, we observe SD down to the nanosecond scale. We quantify SD over 9 orders of magnitude in the time domain from single measurement and show that it follows a power-law dynamics analogous to a random walk. Furthermore, analysing SD in single CsPbBr₃ nanocrystals at cryogenic conditions reveals an unexpected type of fluctuation. Upon increasing the laser power excitation, we observe rapid (sub millisecond) changes in the number of spectral peaks. Namely, two spectral features merge into a single one. I will discuss this discovery, explain why we suspect that phase transitions in the lattice symmetry are responsible for it and how we learn from it what is the underlying mechanism of SD in halide-perovskite nanocrystals.

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