Experimental realization of Popper´s experiment

Summary form only given. In early 1930´s, Karl Popper, a metaphysical realist, proposed a thought experiment to support his realistic interpretation of quantum mechanics, which shared the same goal of the EPR gedankenexperiment. Based on the same EPR two-particle entangled state, Popper asked: if the precise position of particle 2 is known due to the precise measurement of the position for its entangled twin particle 1, does particle 2 experience a greater uncertainty in its momentum? This question has been ignored for many years. One may not like Popper´s philosophy of quantum mechanics; however, his question is closely related to an important problem in recent hot applications: what one can predict for the remaining parts of an entangled state if one of the subsystems is measured alone? We have realized Popper´s experiment with the use of entangled two-photon source of spontaneous parametric down conversion. We conclude from this experiment: in an entangled two-particle system, the precise knowledge of the position of a particle, which is determined by the two-particle entanglement, does not necessarily produce an uncertainty in its momentum.