Search for: All records

We develop a systematic framework for the spin adaptation of the cumulants of p-particle reduced density matrices (RDMs), with explicit constructions for p = 1 to 3. These spin-adapted cumulants enable rigorous treatment of both Ŝz and Ŝ2 symmetries in quantum systems, providing a foundation for spin-resolved electronic structure methods. We show that complete spin adaptation—referred to as completeS-representability—can be enforced by constraining the variances of Ŝz and Ŝ2, which require the 2-RDM and 4-RDM, respectively. Importantly, the cumulants of RDMs scale linearly with system size—size-extensive—making them a natural object for incorporating spin symmetries in scalable electronic structure theories. The developed formalism is applicable to density-based methods, one-particle RDM functional theories, and two-particle RDM methods. We further extend the approach to spin–orbit-coupled systems via total angular momentum adaptation. Beyond spin, the framework enables the adaptation of RDM theories to additional symmetries through the construction of suitable irreducible tensor operators. 

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 

A “critical seed” of exciton condensation is found in molecular-scale fragments of van der Waals heterostructure bilayersviathe theoretical signature for exciton condensation, a large eigenvalue in the particle-hole reduced density matrix. 

We explore the simulation of conical intersections (CIs) on quantum devices, setting the groundwork for potential applications in nonadiabatic quantum dynamics within molecular systems.