  Semiconductor spins are one of the few qubit realizations that remain a
serious candidate for the implementation of large-scale quantum circuits.
Excellent scalability is often argued for spin qubits defined by lithography
and controlled via electrical signals, based on the success of conventional
semiconductor integrated circuits. However, the wiring and interconnect
requirements for quantum circuits are completely different from those for
classical circuits, as individual DC, pulsed and in some cases microwave
control signals need to be routed from external sources to every qubit. This is
further complicated by the requirement that these spin qubits currently operate
at temperatures below 100 mK. Here we review several strategies that are
considered to address this crucial challenge in scaling quantum circuits based
on electron spin qubits. Key assets of spin qubits include the potential to
operate at 1 to 4 K, the high density of quantum dots or donors combined with
possibilities to space them apart as needed, the extremely long spin coherence
times, and the rich options for integration with classical electronics based on
the same technology.
