  A no-signalling channel transforming quantum systems in Alice's and Bob's
laboratories is compatible with two different causal structures: (A < B)
Alice's output causally precedes Bob's input and (B< A) Bob's output causally
precedes Alice's input. I show that a quantum superposition of circuits
operating within these two causal structures enables the perfect discrimination
between no-signalling channels that can not be perfectly distinguished by any
ordinary circuit.
