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Fluorescence lifetime measurements using photon pair correlations generated via spontaneous parametric down conversion (SPDC)

Audrey V. Eshun, Xiyu Yi, Ashleigh K. Wilson, Sam Jeppson, Jae‐Hyuck Yoo, Shervin Kiannejad, Mike Rushford, Tiziana Bond, Ted A. Laurence

Published Jul 14, 2023• 15 citations
Publisher

Abstract

We have used photon pair correlations generated via spontaneous parametric downconversion (SPDC) to measure the fluorescence lifetime of the organic dye rhodamine 6 G, demonstrating that fluorescence lifetime measurements can be achieved using a continuous wave (CW) laser, without pulsed or modulated lasers. Our entangled photon method, quantum fluorescence lifetime (Q-FL) measurements, uses one photon to excite fluorescence and the resulting fluorescence photon is timed and referenced to the arrival time of the other entangled photon. Thus, we can exploit the short timescale of photon pair correlations to conduct experiments that are typically carried out with pulsed lasers and we show that the inherent timing of the photons is fast enough to resolve the nanosecond scale fluorescence lifetime of the sample. This measurement paves the way towards using the time correlations of entangled photons for fluorescence imaging; capitalizing on the presence of fast, sub-100 ps correlations that have not been demonstrated classically.

DOI
10.1364/oe.494744

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We have used photon pair correlations generated via spontaneous parametric downconversion (SPDC) to measure the fluorescence lifetime of the organic dye rhodamine 6 G, demonstrating that fluorescence lifetime measurements can be achieved using a continuous wave (CW) laser, without pulsed or modulated lasers. Our entangled photon method, quantum fluorescence lifetime (Q-FL) measurements, uses one photon to excite fluorescence and the resulting fluorescence photon is timed and referenced to the arrival time of the other entangled photon. Thus, we can exploit the short timescale of photon pair correlations to conduct experiments that are typically carried out with pulsed lasers and we show that the inherent timing of the photons is fast enough to resolve the nanosecond scale fluorescence lifetime of the sample. This measurement paves the way towards using the time correlations of entangled photons for fluorescence imaging; capitalizing on the presence of fast, sub-100 ps correlations that have not been demonstrated classically.
Fluorescence lifetime experiments are a standard approach for measuring excited-state dynamics and local environmental effects. Here, we show that entangled photon pairs produced from a continuous-wave (CW) laser diode can replicate pulsed laser experiments without phase modulation. As a proof of principle, picosecond fluorescence lifetimes of indocyanine green are measured in multiple environments. The use of entangled photons has three unique advantages. First, low-power CW laser diodes and entangled photon source design lead to straightforward on-chip integration for a direct path to distributable fluorescence lifetime measurements. Second, the entangled pair's wavelength is easily tuned by adjusting the temperature or electric field, allowing a single source to cover octave bandwidths. Third, femtosecond temporal resolutions can be reached without requiring major advances in source technology or external phase modulation. Entangled photons could therefore provide increased accessibility to time-resolved fluorescence while also opening new scientific avenues in photosensitive and inherently quantum systems.
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
Quantum light sources, such as entangled photons, offers unique advantages for bio-sensing of light sensitive samples, providing background free and high SNR measurements at ultra-low photon flux. We built a modular set-up based on SPDC entangled photon pairs generation by using a pulsed laser and several single-photon detectors with different time resolution and quantum efficiency to investigate fluorescent complexes. This toolkit paves the way for minimally invasive fluorescence lifetime sensing towards next generation in vivo studies.
Fluorescence microscopy has become integral to biological studies for the technique’s ability to elucidate structures of biomolecules for in-situ studies with high selectivity and specificity. Imaging of intrinsic indicators, such as fluorescent amino acids in proteins, provides important information, but can be challenging to accomplish. Current microscopy techniques that measure native fluorescence without the use of exogenous labels involve either direct UV excitation which is commonly non-localized and can be detrimental to the system, or multiphoton absorption which must be conducted at high intensities, therefore posing high risks of photodamage. As such, we seek to investigate an efficient way to gently excite native fluorescence in biological systems in a way that overcomes these limitations. Quantum entangled photon pairs generated via spontaneous parametric downconversion (SPDC), may be an alternative to conducting two-photon absorption (TPA) to excite fluorescence in amino acids without the high fluences currently used. These photon pairs are highly correlated in time. Thus, the arrival of one photon is simultaneously followed by the arrival of its sister photon. As a result, a molecule interacting with the photon pair should simultaneously absorb both photons, leading to a linear two-photon absorption rate, and the linearity of the two-photon process should dramatically reduce the light intensity necessary for TPA. Therefore, quantum entangled photon pairs offer the possibility of performing low intensity UV excitation using photons in the visible wavelength range. With this work, we generated and characterized entangled photons generated via SPDC, and investigated whether fluorescent amino acids can be excited, and the subsequent fluorescence induced with entangled two-photon absorption. Results show that much higher entangled two-photon rates than what are currently available are needed to measure significant signals with entangled two-photon excitation.