We present a fully quantum-mechanical comparison of two-photon absorption (TPA) probabilities for a single pulse among three photon states: an entangled photon-pair state (EPPS), a separable photon-pair state (PPS), and a multimode coherent state (MCS). While the EPPS and PPS are commonly compared in entangled two-photon absorption (ETPA) theories, the MCS, i.e., the quantum-optical description of pulsed laser light, has not been directly included in such comparisons. By focusing on the two-photon component of MCS, we show that the TPA probability for the two-photon component of MCS is strictly bounded above by that for the PPS, with the ratio governed by the Poisson distribution. This establishes a rigorous hierarchy of single-pulse TPA efficiency for $n\lesssim 1$: EPPS $>$ PPS $>$ MCS. The dominant contribution to the efficiency gap between ETPA and pulsed laser TPA originates from the photon statistics, providing a quantum-optical explanation for the enormous difference observed experimentally.

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• Nonlinear optics
• Photon statistics
• Quantum optics