Reading the fleeting moments of photosynthesis at sunlight intensity… correlated-photon spectroscopy demonstrated

Abstract

Using the correlations between photon pairs generated by spontaneous parametric down-conversion (SPDC) from a continuous-wave (CW) laser, researchers have performed time- and frequency-resolved fluorescence spectroscopy at illumination intensities comparable to real-world sunlight. By measuring purified light-harvesting complexes (LH2) and intact photosynthetic membranes, they tracked at the single-photon level how excitation energy transfer lifetimes change with the open or closed state of the reaction center (RC). Wavelength-integrated fluorescence lifetime traces were recorded in as little as 0.6 seconds, though the authors state plainly that this is not a demonstration of quantum advantage in the strict sense of using entanglement to break a classical sensitivity limit. [Quantum Biology Society] Ultrafast optical spectroscopy has played a central role in studying the earliest dynamics of processes such as photosynthesis and vision. Conventional pulsed-laser methods, however, pour an enormous number of photons onto a sample in a vanishingly brief instant, a condition far removed from the natural setting of sunlight, where a pigment absorbs single photons only rarely. Spectroscopy using quantum states of light has been put forward as an alternative, but experimental demonstrations have been scarce and have typically demanded measurements running for hours or more, a serious practical barrier. A collaboration between the University of Glasgow in the UK and the Politecnico di Milano in Italy has now reported a way to close that gap, publishing in Nature Communications in September 2025. ■ Pairing a continuous-wave laser with correlated photons The team focused a 413 nm continuous-wave (CW) laser into a nonlinear crystal, periodically poled KTiOPO4 (ppKTP), and used spontaneous parametric down-conversion (SPDC) to generate entangled photon pairs at 800 and 860 nm. The 860 nm photon was detected by a single-photon avalanche diode (SPAD) and served as the heralding photon, while its 800 nm twin was sent to the sample. Because the arrival times of the two photons are correlated, the pair itself can drive a time-correlated single photon counting (TCSPC) unit, and the experiment reaches a time resolution of 100 to 200 ps with no pulsed laser anywhere in the setup. The herald arm also carries a tunable interference filter, so choosing the herald wavelength effectively chooses the excitation wavelength of its partner, and tuning the crystal temperature shifts the pair further, giving excitation tunability from 670 nm to 1100 nm. A birefringent common-path interferometer (TWINS) placed in front of the fluorescence detector adds spectral resolution on the detection side. Unlike a grating spectrometer feeding a detector array, this Fourier-transform approach collects all the light onto a single pixel. On the authors' estimate, based on their own measurements and published specifications for state-of-the-art SPAD cameras, that is worth a factor of roughly 4 to 8 in signal-to-noise ratio (SNR) for the same total acquisition time. ■ Tracking energy flow in intact photosynthetic membranes The technique was then applied to real photosynthetic proteins. LH2 complexes purified from the purple photosynthetic bacterium Rhodoblastus acidophilus, excited one photon at a time, gave a single-exponential fluorescence lifetime of 1.13 ± 0.01 ns, in excellent agreement with the value measured on the same sample using a classical femtosecond pulsed laser. The researchers went on to analyze intact photosynthetic membranes from Rhodobacter sphaeroides, which contain LH2, LH1 and the reaction center (RC) together. With the RCs open and able to carry out charge separation, excitation energy drains rapidly onward to the RC and the fluorescence lifetime is short: 101 ± 3 ps. When the RCs were closed by adding a reducing agent, charge separation was blocked, the RC equilibrated with LH1, which then became the terminal emitter, and the lifetime lengthened to 248 ± 9 ps, with the emission spectrum red-shifting to peak near 875 nm. Taken together, these measurements capture at the single-photon level the cascade that carries light energy from LH2 through LH1 to the RC. The fastest step in that chain, the roughly 1 ps transfer from B800 to B850 within LH2, remains beyond the current time resolution; what the experiment reads out is the excited-state lifetime that the cascade produces. ■ A dramatic cut in measurement time, and the boundary of quantum advantage The clearest breakthrough in this work is speed. The team reduced the time needed to record a wavelength-integrated TCSPC fluorescence decay to 0.6 seconds while keeping the SNR at about 4, still sufficient to yield a lifetime estimate in line with those from far longer acquisitions. Quantum-light spectroscopy, once a matter of hours, has entered sub-second territory. Full time- and frequency-resolved maps still take longer, since the interferometer must be scanned position by position: 3.3 minutes for the faster LH2 map shown in the paper, and 3.3 hours at the finest sampling. The authors drew a firm line around what the result does and does not show. They state explicitly in the paper that they have not demonstrated quantum advantage in the rigorous sense of using entanglement to surpass a classical bound on sensitivity or resolution. What they do claim is that the correlations of entangled photon pairs make fast, efficient time-resolved fluorescence measurement possible with nothing more than a CW laser, something that cannot be achieved by classical means. The practical implication is that observing photosynthetic machinery under sunlight-like, extremely low-light conditions could settle into a routine laboratory tool rather than remaining a special demonstration. #QuantumBiology #Photosynthesis #SinglePhoton #CorrelatedPhotons #EntangledPhotonPairs #SpontaneousParametricDownConversion #SPDC #FluorescenceLifetime #TCSPC #LH2 #LH1 #ReactionCenter #PurpleBacteria #QuantumLightSpectroscopy #NatureCommunications https://www.nature.com/articles/s41467-025-63830-3