Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Spectroscopy and microscopy schemes that use quantum states of light have been widely described theoretically with fewer experimental demonstrations that typically require very long measurements that can extend for hours or more. Here, we show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time or wavelength tunability, recording a fluorescence lifetime trace in biological samples in less than a second with acceptable signal-to-noise ratio. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal correlation between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral correlation to select the excitation frequency. We also add spectral resolution in detection, using a 'photon-efficient' Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of this approach by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under illumination intensity conditions comparable to real-world sunlight illumination.
Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Spectroscopy and microscopy schemes that use quantum states of light have been widely described theoretically with fewer experimental demonstrations that typically require very long measurements that can extend for hours or more. Here, we show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time or wavelength tunability, recording a fluorescence lifetime trace in biological samples in less than a second with acceptable signal-to-noise ratio. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal correlation between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral correlation to select the excitation frequency. We also add spectral resolution in detection, using a 'photon-efficient' Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of this approach by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under illumination intensity conditions comparable to real-world sunlight illumination.
Classical time-resolved optical spectroscopy experiments are performed using sequences of ultrashort light pulses, with photon fluxes incident on the sample which are many orders of magnitude higher than real-world conditions corresponding to sunlight illumination. Here we overcome this paradigm by exploiting quantum correlations to perform time-resolved spectroscopy with entangled photons. Starting from spontaneous parametric down-conversion driven by a continuous-wave laser, we exploit the temporal entanglement between randomly generated signal/idler pairs to obtain temporal resolution, and their spectral entanglement to select the excitation frequency. We also add spectral resolution in detection, using a Fourier transform approach which employs a common-path interferometer. We demonstrate the potential of our entangled-photon streak camera by resolving, at the single-photon level, excitation energy transfer cascades from LH2 to LH1 in the photosynthetic membrane and disentangling the lifetimes of two dyes in a mixture. We show that time-resolved spectroscopy with quantum light can be performed without compromising measurement time, recording a fluorescence time trace in less than a minute even for samples with low quantum yield, which can be reduced to sub-second times with acceptable signal-to-noise ratio. Our results provide a new approach to ultrafast optical spectroscopy, where experiments are performed under conditions comparable to real-world sunlight illumination.
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
Abstract Two-photon excitation spectroscopy is a nonlinear technique that has gained rapidly in interest and significance for studying the complex energy-level structure and transition probabilities of materials. While the conventional spectroscopy based on tunable classical light has been long established, quantum light provides an alternative way towards excitation spectroscopy with potential advantages in temporal and spectral resolution, as well as reduced photon fluxes. By using a quantum Fourier transform that connects the sum-frequency intensity and N00N-state temporal interference, we present an approach for quantum interferometric two-photon excitation spectroscopy. Our proposed protocol overcomes the difficulties of engineering two-photon joint spectral intensities and fine-tuned absorption-frequency selection. These results may significantly facilitate the use of quantum interferometric spectroscopy for extracting the information about the electronic structure of the two-photon excited-state manifold of atoms or molecules without any requirement for precise and complicated scanning in the spectral domain. This may be particularly relevant for photon-sensitive biological and chemical samples.
Entangled photons have attracted increasing interest as resources for developing time-resolved spectroscopic techniques. Theoretical studies suggest that their non-classical correlations enable time-resolved spectroscopy with monochromatic pumping and can selectively isolate specific Liouville pathways in nonlinear optical signals. In an earlier study, we proposed a fluorescence detection scheme that could, in principle, be implemented using existing single-photon detectors [Y. Fujihashi et al., Sci. Adv. 12, eaed7026 (2026)]. In that design, the time origin was defined by detecting the arrival of the pulsed laser used to pump the nonlinear crystal for spontaneous parametric down-conversion, a requirement that made the overall experiment cumbersome. This study theoretically examines an alternative protocol that defines the reference time based on the arrival of idler photons. We demonstrate that this idler-referenced scheme functions effectively when the entangled photons exhibit either negative or negligible frequency correlations. Eliminating the pump-timing channel simplifies the optical layout and lowers the experimental barrier to realizing time-resolved two-dimensional fluorescence spectroscopy with entangled photons. Although the photons may exhibit frequency correlations in isolation, their frequency-time degrees of freedom can behave as effectively uncorrelated when considered over the full measurement timescale. Therefore, fully exploiting non-classical correlations requires an entangled photon source whose temporal characteristics are carefully matched to the overall timescale of the experiment.
Quantum optics plays a central role in the study of fundamental concepts in quantum mechanics, and in the development of new technological applications. Typical experiments employ sources of photon pairs generated by parametric processes such as spontaneous parametric down-conversion and spontaneous four-wave-mixing. The standard characterization of these sources relies on detecting the pairs themselves and thus requires single photon detectors, which limit both measurement speed and accuracy. Here it is shown that the two-photon quantum state that would be generated by parametric fluorescence can be characterised with unprecedented spectral resolution by performing a classical experiment. This streamlined technique gives access to hitherto unexplored features of two-photon states and has the potential to speed up design and testing of massively parallel integrated nonlinear sources by providing a fast and reliable quality control procedure. Additionally, it allows for the engineering of quantum light states at a significantly higher level of spectral detail, powering future quantum optical applications based on time-energy photon correlations.
Quantum light is a key resource for promoting quantum technology. One such class of technology aims to improve the precision of optical measurements using engineered quantum states of light. In this study, we investigate transmission measurement of frequency-entangled broadband photon pairs generated via parametric down-conversion with a monochromatic laser. It is observed that state-to-state dynamics in the system under study are temporally resolved by adjusting the path difference between the entangled twin beams when the entanglement time is sufficiently short. The non-classical photon correlation enables time-resolved spectroscopy with monochromatic pumping. It is further demonstrated that the signal corresponds to the spectral information along anti-diagonal lines of, for example, two-dimensional Fourier-transformed photon-echo spectra. This correspondence inspires us to anticipate that more elaborately engineered photon states would broaden the availability of quantum light spectroscopy.