  Quantum gates between spin qubits can be implemented leveraging the natural
Heisenberg exchange interaction between two electrons in contact with each
other. This interaction is controllable by electrically tailoring the overlap
between electronic wavefunctions in quantum dot systems, as long as they occupy
neighbouring dots. An alternative route is the exploration of superexchange -
the coupling between remote spins mediated by a third idle electron that
bridges the distance between quantum dots. We experimentally demonstrate direct
exchange coupling and provide evidence for second neighbour mediated
superexchange in a linear array of three single-electron spin qubits in
silicon, inferred from the electron spin resonance frequency spectra. We
confirm theoretically through atomistic modeling that the device geometry only
allows for sizeable direct exchange coupling for neighbouring dots, while next
nearest neighbour coupling cannot stem from the vanishingly small tail of the
electronic wavefunction of the remote dots, and is only possible if mediated.
