Fig. 2: Two-photon Bragg spectra of BEC with tunable interaction.
a Two-photon Bragg spectra for two different scattering lengths (in unit of Bohr radius of a0). Fraction of atomic BEC diffracted into the momentum state p = 2ħk as a function of the frequency shift ∆ω, which is referenced to the noninteracting free-particle energy 4ER/ħ, after 1 ms Bragg pulse. The resonance shift is determined by a Gaussian fit to the data (solid lines). Error bars denote the standard errors. b Dependence of the frequency shift ∆ω extracted from the fitting in (a) on the scattering length a. The grey solid line is obtained by the Bogoliubov theory, and the red solid line is the prediction based on the Feynman-Tan relation in Eq. (4). The averaged density of ρ = 1.6 × 1013/cm3 is taken in the theoretical calculation. Error bars denote the fitting errors
