9. Summary: the qubit platforms compared

The four platforms of this part side by side.

Platform	The qubit	Control	Readout
Spin qubits	The spin of a single electron trapped in a quantum dot in silicon	Microwave pulses and gate voltages	Spin-to-charge conversion, sensed by a nearby charge sensor
Superconducting qubits	Two energy levels of a superconducting circuit with a Josephson junction (the transmon)	Microwave pulses	Through a coupled microwave resonator
NV centres	The electron spin of a nitrogen-vacancy defect in diamond, with nearby nuclear spins as memory	Microwave pulses	Optically, by spin-dependent fluorescence
Topological qubits	Non-local states of Majorana modes in superconducting nanowires	Braiding the modes around one another	Measuring the parity of pairs of modes

Spin and superconducting qubits are built with the tools of the semiconductor and microwave industries, which makes them natural candidates for scaling up. NV centres keep their coherence over a wide range of temperatures and link easily to photons, which suits quantum networks. Topological qubits promise protection against errors from the physics itself, which is why they get the most space here; they are also the least mature.