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Bose-Einstein condensation of excitons, with its potential for frictionless energy transport, has recently been observed in materials at low temperatures. Here, we show that partial exciton condensation plays a significant role in the 18-chromophore B850 ring of the light-harvesting complex 2 (LH2) in purple bacteria. Even in the single-excitation regime, we observe that excitonic entanglement across multiple sites exhibits signatures of exciton condensation in the particle-hole reduced density matrix—a partial exciton condensate. Crucially, we find that, by distributing the exciton across multiple sites of the ring, the exciton-condensate-like state sets favorable conditions for enhanced energy transfer, both before and after decoherence. Surprisingly, this discovery reveals that excitonic condensation, previously thought to require extreme conditions, can occur in a partial form in biological systems under ambient conditions, providing new insight into energy transport. These results additionally bring new insight into the long-standing debate on quantum versus classical mechanisms in photosynthetic light harvesting by showing that quantum coherence, in the form of a partial exciton condensate, indirectly initializes subsequent classical transfer. Our findings not only deepen our understanding of quantum coherence in light harvesting but also suggest design principles for materials capable of leveraging excitonic entanglement for efficient energy transport. Published by the American Physical Society2025
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This content will become publicly available on October 1, 2026
Delocalized Excitation Transfer in Open Quantum Systems with Long-Range Interactions
The interplay between coherence and system-environment interactions is at the basis of a wide range of phenomena, from quantum information processing to charge and energy transfer in molecular systems, biomolecules, and photochemical materials. In this work, we use a Frenkel exciton model with long-range interacting qubits coupled to a damped collective bosonic mode to investigate vibrationally assisted transfer processes in donor-acceptor systems featuring internal substructures analogous to light-harvesting complexes. We find that certain delocalized excitonic states maximize the transfer rate and that the entanglement is preserved during the dissipative transfer over a wide range of parameters. We investigate the reduction in transfer caused by static disorder, white noise, and finite temperature and study how transfer efficiency scales as a function of the number of dimerized monomers and the component number of each monomer, finding which excitonic states lead to optimal transfer. Finally, we provide a realistic experimental setting to realize this model in analog trapped-ion quantum simulators. Analog quantum simulation of systems comprising many and increasingly complex monomers could offer valuable insights into the design of light-harvesting materials, particularly in the nonperturbative intermediate parameter regime examined in this study, where classical simulation methods are resource intensive.
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- PAR ID:
- 10653301
- Publisher / Repository:
- APS
- Date Published:
- Journal Name:
- PRX Quantum
- Volume:
- 6
- Issue:
- 4
- ISSN:
- 2691-3399
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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