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   https://doi.org/10.1364/OE.396960  

   Thiel, Valérian ; Davis, Alex_O_C ; Sun, Ke ; D’Ornellas, Peru ; Jin, Xian-Min ; Smith, Brian_J ( June 2020 , Optics Express)

   Single-photon sources are a fundamental resource in quantum optics and quantum information science. Photons with differing spectral and temporal shapes do not interfere well and inhibit the performance of quantum applications such as linear optics quantum computing, boson sampling, and quantum networks. Indistinguishability and purity of photons emitted from different sources are crucial properties for many quantum applications. The ability to determine the state of single-photon sources therefore provides a means to assess their quality, compare different sources, and provide feedback for source tuning. Here, we propose and demonstrate a single-configuration experimental method enabling complete characterization of the spectral-temporal state of a pulsed single-photon source having both pure and mixed states. The method involves interference of the unknown single-photon source with a reference at a balanced beam splitter followed by frequency-resolved coincidence detection at the outputs. Fourier analysis of the joint-spectral two-photon interference pattern reveals the density matrix of the single-photon source in the frequency basis. We present an experimental realization of this method for pure and mixed state pulsed single-photon sources.

    

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2. Quantum-clustered two-photon walks

   https://doi.org/10.1103/PhysRevA.101.032118  

   Simon, David S. ; Osawa, Shuto ; Sergienko, Alexander V. ( March 2020 , Physical Review A)

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3. Spectrally engineered broadband photon source for two-photon quantum interferometry

   https://doi.org/10.1364/OE.24.024947  

   Thomas, Abu ; Van Camp, Mackenzie ; Minaeva, Olga ; Simon, David ; Sergienko, Alexander V. ( October 2016 , Optics Express)
4. Two-photon polymerization for biological applications

   https://doi.org/10.1016/j.mattod.2017.06.004  

   Nguyen, Alexander K. ; Narayan, Roger J. ( July 2017 , Materials Today)
5. Colors of entangled two-photon absorption

   https://doi.org/10.1073/pnas.2307719120  

   Varnavski, Oleg ; Giri, Sajal Kumar ; Chiang, Tse-Min ; Zeman, Charles J. ; Schatz, George C. ; Goodson, Theodore ( August 2023 , Proceedings of the National Academy of Sciences)

   Multiphoton absorption of entangled photons offers ways for obtaining unique information about chemical and biological processes. Measurements with entangled photons may enable sensing biological signatures with high selectivity and at very low light levels to protect against photodamage. In this paper, we present a theoretical and experimental study of the excitation wavelength dependence of the entangled two-photon absorption (ETPA) process in a molecular system, which provides insights into how entanglement affects molecular spectra. We demonstrate that the ETPA excitation spectrum can be different from that of classical TPA as well as that for one-photon resonant absorption (OPA) with photons of doubled frequency. These results are modeled by assuming the ETPA cross-section is governed by a two-photon excited state radiative linewidth rather than by electron-phonon interactions, and this leads to excitation spectra that match the observed results. Further, we find that the two-photon-allowed states with highest TPA and ETPA intensities have high electronic entanglements, with ETPA especially favoring states with the longest radiative lifetimes. These results provide concepts for the development of quantum light–based spectroscopy and microscopy that will lead to much higher efficiency of ETPA sensors and low-intensity detection schemes. 

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