Decoding quantum transport in disordered systems

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A new study from IISc led by Vivek Tiwari, Associate Professor at the Solid State and Structural Chemistry Unit (SSCU), addresses a central question in quantum transport.

In highly ordered molecular nanotubes, excitons can travel over extended distances even at room temperature. Because of electronic delocalisation, energy movement is fast and efficient. But disordered systems like natural photosynthesis are fundamentally different. At 300K, they are highly disordered, and yet can transport exciton over hundreds of nanometers lengthscales and on picosecond (10^-12 seconds) timescales with near unity quantum efficiency. This simply cannot be explained by incoherent hopping from one molecule to the next. In such systems, strong localisation of electronic wavefunction would naturally be expected such that it should rapidly lose its quantum behaviour (a phenomenon called decoherence). In reality, however, decoherence doesn’t dominate. Why?

In the study, the researchers used porphyrin nanotubes to mimic natural photosynthesis in the lab. They experimentally built and refined advanced optical instrumentation – a femtosecond 2D spectrometer with >300 nm spectral coverage and ~10 fs temporal resolution – to map excitation versus detection energy with ultrafast precision.

The team found that in the nanotubes, on an energy scale of ~1 eV, electronic delocalisation emerges within ~100 femtoseconds – a deeply quantum regime beyond classical transport models. They observed coupled vibronic motion – slow nuclear motions and fast electronic degrees of freedom interacting in a strict “quantum” regime that requires zero-point vibrational motion to operate.

Perhaps most intriguingly, they found that energetic disorder is not merely a localising nuisance. In fact, the energetic disorder acts as a design principle, boosting electronic delocalisation by enabling zero-point motions to assist in delocalising electronic wavefunctions and overcoming the localising effect of disorder.

Quantum transport in complex, disordered systems continues to surprise scientists. These findings suggest that nature may be using disorder as a design principle to make energy transfer robust in noisy environments. Understanding these phenomena could also help us design more efficient solar cells and artificial light-harvesting systems that work reliably in real world conditions.

REFERENCE:
Thomas AS, Roy C, Roy I, Bhat VN, Ghosh S, Tiwari V, Disordered light-harvesting aggregates can host functional vibronic couplings at room temperature, Nature Communications (2026). Editors’ Pick.
https://www.nature.com/articles/s41467-026-69815-0

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