Correlated-photon time- and frequency-resolved optical spectroscopy

By Dr. Noopur JainReviewed by Louis CastelDec 12 2025

A recent article published in the journal Nature Communications demonstrates time-resolved fluorescence spectroscopy using entangled photon pairs from a continuous-wave laser, replacing ultrafast pulses with quantum timing correlations. The approach enables low-light, wavelength-tunable lifetime measurements suitable for sensitive biological samples without sacrificing acquisition speed.

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A correlated photon streak camera for biological spectroscopy

Traditional time-resolved optical measurements usually expose the sample to photon doses far exceeding those encountered under natural illumination, such as sunlight. The study shows that using quantum light for time-resolved spectroscopy preserves both rapid acquisition and wavelength flexibility, allowing fluorescence lifetimes in biological specimens to be captured in under a second with sufficient SNR. This method uses entangled photon pairs (EPPs) generated by spontaneous parametric down-conversion (SPDC) driven by a continuous-wave (CW) laser. This method makes use of the time correlation of the randomly produced signal–idler photon pairs to achieve temporal resolution, while their spectral correlation enables the selection of the excitation wavelength. Additionally, spectral resolution is achieved in detection using a "photon-efficient" Fourier transform approach that employs an ultra-stable birefringent interferometer. The combination of temporal and spectral resolution is referred to as a "correlated photon streak camera”.

The Current Study

The experimental setup begins with a CW single-frequency blue laser (lambda = 413 nm) focused into a periodically poled KTiOPO₄ (ppKTP) crystal to generate EPPs at 800 nm (signal photon) and 860 nm (idler/heralding photon) via type 0 quasi phase matching. This SPDC process provides the temporal and spectral correlations that are the foundation of the technique.

The 860-nm heralding photon is spectrally filtered and coupled via a single-mode fiber to a single-photon avalanche diode (SPAD). The 800-nm signal photon is used to excite the molecular sample. Fluorescence from the sample is collected in the back-scattering direction and coupled via a multi-mode fiber to a second SPAD. The outputs from the two SPADs are sent to a TCSPC unit, which monitors the delay of the fluorescence photon relative to the heralding photon, thereby measuring the fluorescence lifetime. The detection of the fluorescence photon is conditioned, or "heralded," both spectrally and temporally by the idler photon. Furthermore, the heralding photon is filtered to ensure that fluorescence is collected only from events triggered by pump (signal) photons with the desired frequency, enabling simultaneous conditioning in time and frequency of the pump photon.

To add spectral resolution to the detection path, the authors insert a TWINS interferometer (manufactured by NIREOS, a company with which one of the authors, G.C., has a financial association). This interferometer is a common-path Fourier transform device.

Spectrally resolved fluorescence lifetimes with entangled photons

The authors demonstrate that TCSPC using EPPs can achieve data quality comparable to classical measurements in terms of temporal resolution and SNR, but without significant compromises in measurement time and while dramatically decreasing the excitation fluence. The current temporal resolution is limited to approximately 100–200 ps by the instrumental response function of the photodetectors, although this could be improved to sub-picosecond levels by exploiting the Hong-Ou-Mandel (HOM) effect.

A key result is the achievement of spectrally resolved lifetime measurements. This capability was demonstrated by measuring a mixture of two dyes (800CW and IR143). Both dyes absorb at 800 nm but have different emission spectra (peaking at 810 nm and 900 nm) and distinct lifetimes. Using the TWINS interferometer and Fourier transform approach, the authors successfully disentangled the two dyes' lifetimes and their respective emission spectra in the mixture. The total measurement time for the time- and frequency-resolved map was 120 minutes, necessary due to recording multiple time traces for various interferometer wedge positions.

The broad applicability of the approach was also demonstrated by recording photoinduced dynamics of the light-harvesting 2 (LH2) complex. The technique was able to capture a fluorescence time profile of LH2 in under a minute, and even a 1-second acquisition provided sufficient SNR, demonstrating that the approach maintains measurement speed comparable to conventional methods. The authors emphasize that while they exploit correlations from entangled photons for efficient and fast detection of fluorescence lifetime signals with a CW laser source, they are not exploiting entanglement in the sense of violating classical sensitivity or resolution bounds in this specific demonstration.

Conclusion

This work successfully utilizes the temporal and spectral correlations of entangled photon pairs generated by a CW laser-driven SPDC process to perform time- and frequency-resolved fluorescence spectroscopy. The technique achieves efficient and fast detection of fluorescence lifetime signals, recording traces in sub-second acquisition times with an acceptable SNR. This method overcomes the reliance on high-intensity pulsed lasers, enabling experiments to be performed under illumination intensity conditions comparable to real-world sunlight. The capacity to simultaneously measure lifetime and emission spectrum, alongside the rapid acquisition times, establishes correlated-photon spectroscopy as a powerful new tool for ultrafast optical spectroscopy.