Mermin was the first to note how the three-particle GHZ state demonstrates that no local hidden-variable theory can explain quantum correlations, and together with Asher Peres, he introduced the "magic square" proof, another demonstration that attempting to "complete" quantum mechanics with hidden variables does not work. Richard Feynman described another paper by Mermin in this area as "one of the most beautiful papers in physics". In collaboration with Charles Bennett and Gilles Brassard, he made a significant early contribution to quantum cryptography. Starting in 2012, he has advocated the interpretation of quantum mechanics known as Quantum Bayesianism, or QBism.
