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In this work we study the first step in photosynthesis for the limiting caseof a single photon interacting with photosystem II (PSII). We model our systemusing quantum trajectory theory, which allows us to consider not only theaverage evolution, but also the conditional evolution of the system givenindividual realizations of idealized measurements of photons that have beenabsorbed and subsequently emitted as fluorescence. The quantum nature of thesingle photon input requires a fully quantum model of both the input and outputlight fields. We show that PSII coupled to the field via three collective``bright states'', whose orientation and distribution correlate strongly withits natural geometry. Measurements of the transmitted beam strongly affects thesystem state, since a (null) detection of the outgoing photon confirms that thesystem must be in the electronic (excited) ground state. Using numerical andanalytical calculations we show that observing the null result transforms astate with a low excited state population $O( 10^{-5} )$ to a state with nearlyall population contained in the excited states. This is solely a property ofthe single photon input, as we confirm by comparing this behavior with that forexcitation by a coherent state possessing an average of one photon, using asmaller five site ``pentamer'' system. We also examine the effect of adissipative phononic environment on the conditional excited state dynamics. Weshow that the environment has a strong effect on the observed rates offluorescence, which could act as a new photon-counting witness of excitoniccoherence. The long time evolution of the phononic model predicts anexperimentally consistent quantum efficiency of 92%.

A Quantum trajectory picture of single photon absorption and energy transport in photosystem II