We provide a proof of existence of quantum weak coin flipping protocol with arbitrarily small bias. This proof is based on the work of Mochon, while some parts of it are simplified. Moreover, we provide an analysis of the number of qubits and rounds needed in the protocol as a function of the bias. This is joint work with D. Aharonov, A. Chailloux, M. Ganz and L. Magnin.