  An entangled pair of photons (1 and 2) are emitted to opposite directions. A
narrow slit is placed in the path of photon 1 to provide precise knowledge of
its position on the $y$ axis and this also determines the precise $y$ position
of its twin, photon 2, due to quantum entanglement. Is photon 2 going to
experience a greater uncertainty in momentum, i.e., a greater $\Delta p_{y}$,
due to the precise knowledge of its position $y$? The experimental data shows
$\Delta y\Delta p_{y}<\hbar $ for photon 2. Can this recent realization of the
historical thought experiment of Karl Popper signal a violation of the
uncertainty principle?
