Homodyne X-ray diffraction signals produced by classical light and classical detectors are given by the modulus square of the charge density in momentum space ∣σ(q)∣2, missing its phase which is required in order to invert the signal to real space. We show that quantum detection of the radiation field yields a linear diffraction pattern that reveals σ(q) itself, including the phase. We further show that repeated diffraction measurements with variable delays constitute a novel multidimensional measure of spontaneous charge-density fluctuations. Classical diffraction, in contrast, only reveals a subclass of even-order correlation functions. Simulations of two dimensional signals obtained by two diffraction events are presented for the amino acid cysteine.
@article{arxiv.1901.10093,
title = {Monitoring Spontaneous Charge-density Fluctuations by Single-molecule Diffraction of Quantum Light},
author = {Konstantin E. Dorfman and Shahaf Asban and Lyuzhou Ye and Daeheum Cho and Jérémy R. Rouxel and Shaul Mukamel},
journal= {arXiv preprint arXiv:1901.10093},
year = {2019}
}