Discussions regarding the fundamentals of quantum mechanics tend to be philosophical in nature. As physicists it can be good to have in mind what can be actually observed with the currently available technology. In this talk I will present two experiments that investigated a common theme, the trajectory of a photon, from different perspective. The first one [1] is based on a new measurement protocol, utilizing the frequency degree of freedom, which yields an operational definition for the trajectory of a single photon. The resulting trajectories are discontinuous and agree with a conceptual definition coming from the two-state vector formalism. The second experiment [2] depict the Bohmian trajectory of a photon that is entangled with another one. The results clearly show how a measurement on the additional photon instantaneously change the trajectory of the first one. Looking at the two experiments side by side we have a vivid demonstration of the hard choice that quantum mechanics forces us to make: either a realistic trajectory with nonlocal effects or a local theory with trajectories that do not comply with an intuitive (classical) notion of what a trajectory is.

[1] Zong-Quan Zhou, Xiao Liu, YK, Jin-Min Cui, Zong-Feng Li, Yi-Lin Hua, Chuan-Feng Li and Guang-Can Guo, “Experimental observation of anomalous trajectories of single photons”, Physical Review A 95 (4), 042121
[2] Ya Xiao, YK, Jin-Shi Xu, Chuan-Feng Li and Guang-Can Guo “Experimental nonlocal steering of Bohmian trajectories”, Optics express 25 (13), 14463-14472