Fig. 3

Spatial properties of the generated biphoton state. a All Stokes–anti-Stokes coincidences obtained from 10⁷ frames marked with their k _y wavevector components (k _x for the inset) for zero memory storage time, demonstrating high degree of momenta anti-correlation. Each plot disregards the perpendicular component. b Same coincidences counted in the center-of-mass variables (k _x,S + k _x,AS) and (k _y,S + k _y,AS). The central peak is fitted with a two-dimensional Gaussian to obtain its center and width. One-dimensional distributions correspond to cross-section counts selected for central pixels. Both in a, b, neither accidental nor noise background subtraction is performed