Convolution algebra of superoperators and nonseparability witnesses for quantum operations.

We define a product between quantum superoperators which is preserved under the Choiâ€"JamioĹ‚kowskiâ€"Krausâ€"Sudarshan channel-state isomorphism. We then identify the product as the convolution on the space of superoperators, with respect to which the channel-state duality is also an algebra isomorphism. We find that any witness operator for detecting nonseparability of quantum operations on separated parties can be written entirely within the space of superoperators with the help of the convolution product. [ABSTRACT FROM AUTHOR]