Kavli Affiliate: K. Birgitta Whaley
| First 5 Authors: Robert L. Cook, Liwen Ko, K. Birgitta Whaley, ,
| Summary:
In this work we study the first step in photosynthesis for the limiting
extreme case of a single photon interacting with photosystem II (PSII). We
model our system using quantum trajectory theory, which allows us to consider
not only the average evolution, but also the conditional evolution of the
system given individual realizations of idealized measurements of photons that
have been absorbed and subsequently emitted by fluorescence. The quantum nature
of the single photon input requires a fully quantum model of both the input and
output light fields. We show that PSII coupled to the field via three
collective "bright states", whose orientation and distribution correlate
strongly with its natural geometry. Measurements of the transmitted beam
strongly affects the system state, since a (null) detection of the outgoing
photon confirms that the system must be in the electronic (excited) ground
state. Using numerical and analytical calculations we show that observing the
null result transforms a state with a low excited state population $O( 10^{-5}
)$ to a state with nearly all population contained in the excited states. This
is solely a property of the single photon input, as we confirm by comparing
this behavior with that for excitation by a coherent state possessing an
average of one photon, using a smaller five site "pentamer" system. We further
analytically predict and also numerically verify that the time-dependent
variations in the observed rates of fluorescence reflect interference between
eigenstates of the non-Hermitian Hamiltonian that are superposed in the
absorption of the incident single photon, providing a new photon-counting
witness of excitonic coherence in electronic energy transfer.
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