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journal club

**Measuring exciton diffusion rate, Auger and singlet-singlet annihilation rates, and the true number of chromophores using picosecond time-resolved photon antibunching (Dekel Nakar)**

The photon statistics of fluorescence from single quantum systems (single chromophores) shows photon antibunching. In multichromophoric systems, exciton diffusion and subsequent annihilation occurs. These processes also yield photon antibunching but cannot be interpreted reliably. Here the authors develop picosecond time-resolved antibunching to identify and analyze such processes. This method is first used on well-defined multichromophoric DNA-origami structures to precisely determine the distance-dependent rates of annihilation between excitons. Then, this allows to measure exciton diffusion in mesoscopic conjugated-polymer aggregates with different spatial ordering (H- vs. J-type conjugation). The authors distinguish between one-dimensional intra-chain and three-dimensional inter-chain exciton diffusion at different times after excitation and determine the disorder-dependent diffusion lengths. Overall, using this method, excitons can be studied at the single-particle level, enabling the rational design of improved excitonic probes such as ultra-bright fluorescent nanoparticles and materials for optoelectronic devices.

Hedley, G.J., …, Jan Vogelsang et al. Picosecond time-resolved photon antibunching measures nanoscale exciton motion and the true number of chromophores. Nat Commun 12,1327 (2021). https://doi.org/10.1038/s41467-021-21474-z

**Pauli blocking of atom-light scattering (Eran Reches) **

Fermi’s golden rule reveals that the transition rate between two coupled states depends on the density of final states. It is well-known, for instance, that a resonant cavity can enhance the spontaneous emission rate of an atom by increasing the density of states of light. Similarly, reducing the density of final momentum modes of the atomic motion is expected to suppress the rate of radiative processes. This can happen for fermionic atoms embedded in a Fermi sea via the Pauli exclusion principle, which forbids final momentum modes already occupied by other atoms.

In my talk I will present the work in Ref. [1], where the authors experimentally demonstrate the suppression of light scattering rates in a quantum degenerate Fermi gas of strontium atoms by up to a factor of 2, compared with a thermal gas. I will also compare key aspects of their experiment to the work of other groups [2,3].

[1] Sanner, C., Sonderhouse, L., Hutson, R. B., Yan, L., Milner, W. R., & Ye, J. (2021). Pauli blocking of atom-light scattering. Science, 374(6570), 979–983. https://doi.org/10.1126/science.abh3483

[2] Deb, A. B., & Kjærgaard, N. (2021). Observation of Pauli blocking in light scattering from quantum degenerate fermions. Science, 374(6570), 972–975. https://doi.org/10.1126/science.abh3470

[3] Margalit, Y., Lu, Y.-K., Top, F. Ç., & Ketterle, W. (2021). Pauli blocking of light scattering in degenerate fermions. Science, 374(6570), 976–979. https://doi.org/10.1126/science.abi6153

Zoom:https://weizmann.zoom.us/j/99871493260?pwd=K01BTGkwVWFRUzFQQjBTb2VIZ01xdz09