Photon Correlations Control Quantum Fisher Information in Coherent Multidimensional Spectroscopy
Chenghong Huang, Yuta Fujihashi, Akihito Ishizaki
Coherent multidimensional spectroscopy probes interactions and dynamics of molecular systems through nonlinear optical signals. Quantum light excitation has been explored as a means of controlling these signals, raising the question of how excitation-light correlations affect the precision of molecular parameter estimation from the emitted signal. In this work, we show that photon-number correlations and two-photon coherence play distinct roles in encoding molecular information in the emitted rephasing signal. In the weak-emission regime and within the single Schmidt pair approximation, we derive a normally ordered information law in which the joint photon-number moment and the squared magnitude of the two-photon coherence weight the amplitude and phase sensitivities of the molecular response, respectively. For matched temporal modes and mean photon numbers in each input mode, two-mode squeezed vacuum enhances both weights relative to coherent excitation, whereas independent displaced squeezing preserves the amplitude weight but reduces the phase weight. The background population and its parameter dependence also affect the total quantum Fisher information, which therefore cannot be inferred from the coherent mean-field signal alone. We derive a leading-order bound on its enhancement over coherent excitation when the background population is nonnegative and common to the compared sources. These findings provide criteria for selecting excitation sources according to how the target molecular parameter affects the amplitude and phase of the response.
quant-pharXiv:2610.29422•Oct 22, 2026•0 citations