We show that an important class of quantum channels, the convex closure of the spin channels, can be algebraically represented by classical channels that have four inputs and four outputs. This result is used to develop an experimentally realizable scheme for randomly flipping qubits whose basis of preparation is unknown. This scheme can be used to interrupt, but not necessarily eradicate, any form of hidden communication based on quantum information. It can also be used to remove steganographic information embedded in quantum data.
Martin, K. (2011). How to randomly flip a quantum bit. In Electronic Notes in Theoretical Computer Science (Vol. 270, pp. 81–97). https://doi.org/10.1016/j.entcs.2011.01.008