Wigner's friend and continuous time evolution of the state of a closed system and the probabilistic, discontinuous collapse of the state of a system upon measurement. Wigner's friend is therefore directly linked to the measurement problem in quantum mechanics with its famous Schrödinger's cat paradox. The thought experiment posits a friend of Wigner in a laboratory and lets him perform a quantum measurement on a physical system (this could be a spin system or also Schrödinger's cat). This system is assumed to be in a superposition of two distinct states, say, state 0 and state 1 (or "dead" and "alive", in the
Popper's experiment Popper's experiment based on this impossibility of superluminal communication in quantum mechanics. Use of quantum correlations for faster-than-light communication is thought to be flawed because of the no-communication theorem in quantum mechanics. However the theorem is not applicable to this experiment. In this experiment, the "sender" tries to signal 0 and 1 by narrowing the slit, or widening it, thus changing the probability distribution among the "receiver's" detectors. If the no-communication theorem were applicable, then no matter if the sender widens the slit or narrows it, the receiver should see the same probability distribution among his detectors. This is true,