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1. Positive operator-valued measure for two-photon detection via sum-frequency generation

   https://doi.org/doi.org/10.1103/PhysRevA.103.043711  

   Merkouche, Sofiane; Thiel, Valérian; and Smith, Brian J. (January 2021, Physical review and Physical review letters index)

   null (Ed.)

   Spontaneous parametric down conversion (PDC), in the perturbative limit, can be considered as a probabilistic splitting of one input photon into two output photons. Conversely, sum-frequency generation (SFG) implements the reverse process of combining two input photons into one. Here we show that a single-photon projective measurement in the temporal-mode basis of the output photon of a two-photon SFG process affects a generalized measurement on the input two-photon state. We describe the positive operator-valued measure (POVM) associated with such a measurement and show that its elements are proportional to the two-photon states produced by the time-reversed PDC process. Such a detection acts as a joint measurement on two photons and is thus an important component of many quantum information processing protocols relying on photonic entanglement. Using the retrodictive approach, we analyze the properties of the two-photon POVM that are relevant for quantum protocols exploiting two-photon states and measurements. 

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2. Room-Temperature Photon-Number-Resolved Detection Using A Two-Mode Squeezer

   https://doi.org/10.1364/FIO.2018.JW3A.70  

   Matekole, Elisha S.; Vaidyanathan, Deepti; Arai, Kenji W.; Glasser, Ryan T.; Lee, Hwang; Dowling, Jonathan P. (January 2018, Frontiers in Optics)

   We study the average intensity-intensity correlations signal at the output of a two-mode squeezing device with |𝑁𝑁⟩⊗|𝛼𝛼⟩ as the two input modes. We show that the input photon-number can be resolved from the average intensity-intensity correlations. In particular, we show jumps in the average intensity-intensity correlations signal as a function of input photon-number N. Therefore, we propose that such a device may be deployed as photon-number resolving detector at room temperature with high efficiency. 

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3. Measurement of three-photon excitation cross-sections of fluorescein from 1154 nm to 1500 nm

   https://doi.org/10.1364/BOE.498214  

   LaViolette, Aaron_K; Ouzounov, Dimitre_G; Xu, Chris (July 2023, Biomedical Optics Express)

   Measurements of three-photon action cross-sections for fluorescein (dissolved in water, pH ∼11.5) are presented in the excitation wavelength range from 1154 to 1500 nm in ∼50 nm steps. The excitation source is a femtosecond wavelength tunable non-collinear optical parametric amplifier, which has been spectrally filtered with 50 nm full width at half maximum band pass filters. Cube-law power dependance is confirmed at the measurement wavelengths. The three-photon excitation spectrum is found to differ from both the one- and two-photon excitation spectra. The three-photon action cross-section at 1154 nm is more than an order of magnitude larger than those at 1450 and 1500 nm (approximately three times the wavelength of the one-photon excitation peak), which possibly indicates the presence of resonance enhancement. 

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4. Localized Momentum-Space Distribution of Photocurrents Injected by Quantum Interference of Nonlinear Absorption Processes

   https://doi.org/10.1364/FIO.2023.JTu5A.32  

   Gong, Yiming; Wang, Kai; Cundiff, Steven T. (January 2023, Optica Publishing Group)

   Based on polarization dependence, we show that momentum-space distribution of photocurrents is more localized in quantum interference of two- and three-photon absorptions than that in quantum interference of one- and two-photon absorptions. 

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5. Simultaneous Two- and Three-Photon Deep Imaging of Autofluorescence in Bacterial Communities

   https://doi.org/10.3390/s24020667  

   Fernández, Alma; Classen, Anton; Josyula, Nityakalyani; Florence, James T; Sokolov, Alexei V; Scully, Marlan O; Straight, Paul; Verhoef, Aart J (January 2024, Sensors)

   The intrinsic fluorescence of bacterial samples has a proven potential for label-free bacterial characterization, monitoring bacterial metabolic functions, and as a mechanism for tracking the transport of relevant components through vesicles. The reduced scattering and axial confinement of the excitation offered by multiphoton imaging can be used to overcome some of the limitations of single-photon excitation (e.g., scattering and out-of-plane photobleaching) to the imaging of bacterial communities. In this work, we demonstrate in vivo multi-photon microscopy imaging of Streptomyces bacterial communities, based on the excitation of blue endogenous fluorophores, using an ultrafast Yb-fiber laser amplifier. Its parameters, such as the pulse energy, duration, wavelength, and repetition rate, enable in vivo multicolor imaging with a single source through the simultaneous two- and three-photon excitation of different fluorophores. Three-photon excitation at 1040 nm allows fluorophores with blue and green emission spectra to be addressed (and their corresponding ultraviolet and blue single-photon excitation wavelengths, respectively), and two-photon excitation at the same wavelength allows fluorophores with yellow, orange, or red emission spectra to be addressed (and their corresponding green, yellow, and orange single-photon excitation wavelengths). We demonstrate that three-photon excitation allows imaging over a depth range of more than 6 effective attenuation lengths to take place, corresponding to an 800 micrometer depth of imaging, in samples with a high density of fluorescent structures. 

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