The three sets of coloured dots show the measured data at three different incident photon rates represented by the average photon pair number n p generated per pump pulse over a fourfold range (that is, 0.0458 (rate 1, blue), 0.0214 (rate 2, red) and 0.0103 (rate 3, orange), respectively, with 200-s integration time) (Supplementary Tables 2 and 3). c, Normalized coincidence counts of crosscorrelation between heralds and heralded fluorescent photons plotted as a function of their relative time delay τ with 128-ps bin size. b, Schematic of the raw signals showing the relative time delay τ between corresponding pairs of heralds and heralded fluorescent photons. BBO, barium borate PPKTP, periodically poled potassium titanyl phosphate |G>, ground state |1EM>, one-exciton manifold. The inset is a simplified energy diagram of LH2 showing the whole process from absorption of a single photon by the B800 ring to fluorescence of a single photon by the B850 ring after electronic energy transfer from B800. For simplicity, the carotenoids and protein subunits of LH2 are not shown here. In the LH2 structure, the B800 ring (containing 9 bacteriochlorophylls) and the B850 ring (containing 18 bacteriochlorophylls) are colour coded as blue and red, respectively, produced from the Protein Data Bank file 1NKZ using ChimeraX. Finally, each of the detection events in each detector was time tagged with subnanosecond time resolution.Ī, Simplified schematic of the time-resolved PCQLS for studying single-photon transitions in photosynthetic systems. After subsequent energy transfer and relaxation, a fluorescent photon emitted from the B850 ring was then collected, selected through spectral filters and detected by Detector 2. One of the photons, the herald, was directly detected by a single-photon counting module, Detector 1, and signifies the presence of the other photon, which is focused by a home-built microscope into a solution of LH2 complexes under ambient conditions (Supplementary Fig. 2) as confirmed by measurements of the second-order coherence function at zero time delay, g (2)( t = 0), to be discussed later. The probability of having two simultaneously created pairs was maintained at a very small value (Supplementary Information Section II and Supplementary Fig. 1), with photon pairs around 808 nm produced from type II spontaneous parametric down-conversion in a nonlinear crystal pumped by a femtosecond laser with a repetition rate R r = 75.7 MHz. 1, Supplementary Information Section I and Supplementary Fig. Our measurements use time-resolved photon-counting quantum light spectroscopy (PCQLS) based on a heralded single-photon source 20, 21 and coincidence counting (Fig. An analytical stochastic model and a Monte Carlo numerical model capture the data, further confirming that absorption of single photons is correlated with emission of single photons in a natural light-harvesting complex. We also find that the probability distribution of the number of heralds per detected fluorescence photon supports the view that a single photon can upon absorption drive the subsequent energy transfer and fluorescence emission and hence, by extension, the primary charge separation of photosynthesis. Using a heralded single-photon source 20, 21 along with coincidence counting, we establish time correlation functions for B800 excitation and B850 fluorescence emission and demonstrate that both events involve single photons. Excitation of the B800 ring leads to electronic energy transfer to the B850 ring in approximately 0.7 ps, followed by rapid B850-to-B850 energy transfer on an approximately 100-fs timescale and light emission at 850–875 nm (refs. Here, we use single photons to excite under ambient conditions the light-harvesting 2 (LH2) complex of the purple bacterium Rhodobacter sphaeroides, comprising B800 and B850 rings that contain 9 and 18 bacteriochlorophyll molecules, respectively. Yet much experimental and theoretical work over the past 40 years has explored the events during photosynthesis subsequent to absorption of light from intense, ultrashort laser pulses 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Photosynthesis is generally assumed to be initiated by a single photon 1, 2, 3 from the Sun, which, as a weak light source, delivers at most a few tens of photons per nanometre squared per second within a chlorophyll absorption band 1.
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