The intrinsic quantum uncertainty between conjugate observables is a hallmark of quantum theory. Heisenberg's uncertainty has been experimentally tested in elementary particles, molecules and microscopic objects. Uncertainties obtained from diffraction setups have the drawback that position and momentum are measured at different times in dissimilar regions. Joint measurements with alternative setups, have been performed with other complementary variables. A quantum optical realization of the position - linear momentum uncertainty in the quantum limit is presented here. The interference of two non-collinear photon modes with different frequencies are space and time resolved. Detection is performed in the same space-time region, thus achieving a joint measurement. Evaluation of the photon momentum from the position versus time interferograms, makes this procedure akin to the mechanical momentum construct. The obstruction that the position precision imposes on the momentum determination is directly visualized. The measured uncertainties ΔxΔpx∼2ħ, consistent with Hesienberg's principle, discern between two theoretical proposals.