Engineering quantum interactions with confined light

Speaker: Dr Rakesh Arul (Cambridge)
Series: Theory of Condensed Matter

What determines the state of a material when light, electronic excitations, vibrations and spins become inseparable parts of the same problem? Molecular nanophotonics offers a setting in which interactions, optical driving and dissipation jointly select collective behaviour. The central object is a driven, non-equilibrium state whose correlations depend on both the material and its electromagnetic environment. Chemical synthesis provides unusual freedom to tune this competition through electronic structure, exchange coupling, and molecular geometry.

I will first demonstrate how extreme optical confinement produces collective dipole synchronisation in self-assembled plasmonic nanocavity networks at room temperature [1]. Molecular excitons confined within subnanometre gaps between gold nanoparticles develop long-range spatial coherence, despite disorder and ultrafast photon loss. However, spatial phase locking coexisting with temporal photon coherence that decays within approximately 10 femtoseconds, providing an experimental setting for many-body physics under strong driving and dissipation. This results in an unusual physical system which is spatially coherent, but temporally incoherent.

I will then discuss routes to engineer dipole-dipole interactions involving lanthanide 4f excitations and to control spin interactions in molecular spin-bearing systems [2]. These systems offer complementary handles on energy transfer, spin selection rules and optical readout. We show how interfacial spin exchange enables usually spin-forbidden triplet excitations and room-temperature phosphorescence. This unlocks new handles to tune the optoelectronic properties of materials for new LEDs and devices.

Building on these capabilities, I will outline experiments within our new NanoSPINEC project to test entanglement between triplets generated by singlet fission, combining independent spin manipulation with calibrated, spin-selective reaction readout. A related goal is chemically programmable quantum sensing: integrating a molecular spin, optical reporter and binding site to detect nearby spins and local interfacial dynamics. Together, these directions pose a common theoretical challenge: predicting how microscopic interactions, environmental fluctuations and measurement determine which quantum correlations survive, and whether those correlations can control exciton transport, chemical reactivity and sensing.

[1] Arul, R., Fowler-Wright, P., Børresen, L. et al. Coherent room-temperature dipole synchronization in plasmonic nanocavities. Nature Nanotechnology 21, 1230–1237 (2026).

[2] Zhu, H., Arul, R., Jiang, Z. et al. Lanthanide-doped nanocrystals enable organic room-temperature phosphorescence in solution through direct triplet excitation. Nat. Chem. 18, 1242–1248 (2026).