In 1945, he moved to University of Cambridge, and worked as a research student under Paul Dirac. While at Cambridge, he attended lectures by Wolfgang Pauli, and during one of them, Mehrotra pointed out a mistake in Pauli's work. The two became lifelong friends. During this time he became increasingly interested in mathematics. He obtained his PhD, Infinite Irreducible Representations of the Lorentz Group, at Cambridge in 1947 under Dirac.