  We introduce a qubit-based version of the quantum switch, consisting in a
variation of the Fermi problem. Two qubits start in a superposition state where
one qubit is excited and the other in the ground state, but it is undefined
which is the excited qubit. Then, after some time, if a photon is detected, we
know that it must have experienced an emission by one atom and then an
absorption and re-emission by the other one, but the ordering of the emission
events by both qubits is undefined. While it is tempting to refer to this
scenario as one with indefinite causality or a superposition of causal orders,
we show that there is still a precise notion of causality: the probability of
excitation of each atom is totally independent of the other one when the times
are short enough to prevent photon exchange.
