  The scalar product of two vectors with $K$ real components can be computed
using two quantum channels, that is, information transmission lines in the form
of spin-1/2 XX chains. Each channel has its own $K$-qubit sender and both
channels share a single two-qubit receiver. The $K$ elements of each vector are
encoded in the pure single-excitation initial states of the senders. After time
evolution, a bi-linear combination of these elements appears in the only matrix
element of the second-order coherence matrix of the receiver state. An
appropriate local unitary transformation of the extended receiver turns this
combination into a renormalized version of the scalar product of the original
vectors. The squared absolute value of this scaled scalar product is the
intensity of the second-order coherence which consequently can be measured, for
instance, employing multiple-quantum NMR. The unitary transformation generating
the scalar product of two-element vectors is presented as an example.
