Date of Award

2014

Document Type

Honors Thesis (Open Access)

Department

Colby College. Physics and Astronomy Dept.

Advisor(s)

Yoshihiro Sato

Abstract

Long-lived oscillations lasting up to 1800 fs have been observed in the Fenna--Mathews--Olsen (FMO) Pigment-Protein complex. It is unclear if the oscillations are quantum or classical in origin. Quantized intra-pigment normal modes are responsible for these long-lived oscillations. In this thesis we simulate a bacteriachlorophyll dimer in the FMO Complex at physiological conditions. Our model describes two electronically coupled pigments coupled to the vibrational protein environment. Our goal is to find system parameters that allow long-lived quantum coherences to exist in photosynthetic complexes. We calculate the time evolution of our system's density matrix using the numerically exact quantum adiabatic path integral (QUAPI). The coherence of the system was determined by taking the real component of the density matrices complex off-diagonals. The quantumness of the system was determined by taking the Hilbert-Schmidt distance between the density matrix and its corresponding pointer states. By varying the Huang-Rhys factor, the reorganization energy difference, and the site energy difference we found long-lived coherences with an amplitude of 0.15 and a quantumness of 0.23. The quantum coherence was still present 2500 fs after the initial excitation of the system. This suggests that coherent quantum superpositions can theoretically occur in the FMO Complex at physiological conditions. The period of the coherent oscillations were on the same order of magnitude as the time resolution of our simulation causing aliasing to occur. In order to confirm the observed coherent superpositions we need to increase the temporal resolution of the simulation.

Keywords

coherence, quauntumness, FMO, photosynthesis, exciton, biophysics

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