Nonperturbative response functions: A tool for the interpretation of four-wave-mixing signals beyond third order

Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center which stores energy from the photon in chemical bonds. This process occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer - a wavelike transfer mechanism - occurs in many photosynthetic pigment-protein complexes (1-4). Using the Fenna-Matthews-Olson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies further show that this process is equivalent to a quantum random walk algorithm (5-8). This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present the first evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to perform a rudimentary quantum computational operation. This data proves that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations. The persistence of quantum coherence in a dynamic, disordered system under these conditions suggests a new biomimetic strategy for designing dedicated quantum computational devices that can operate at high temperature.

A method is presented for controlling the outcome of photochemical reactions by using the dynamic Stark effect due to a strong, nonresonant infrared field. The application of a precisely timed infrared laser pulse reversibly modifies potential energy barriers during a chemical reaction without inducing any real electronic transitions. Dynamic Stark control (DSC) is experimentally demonstrated for a nonadiabatic photochemical reaction, showing substantial modification of reaction channel probabilities in the dissociation of IBr. The DSC process is nonperturbative and insensitive to laser frequency and affects all polarizable molecules, suggesting broad applicability.