This chapter focuses on the physical platforms that implement quantum information processing. It moves from the criteria a platform must satisfy, through a survey of qubit modalities and the engineering of an actual chip, to how devices are benchmarked and where the field stands.
Two layers are kept deliberately separate. How each platform encodes and manipulates a qubit is evergreen physics and forms the bulk of the chapter. The state of the field at any given moment — device sizes, fidelities, company roadmaps, market figures — is perishable, so it is confined to clearly date-stamped snapshot notes that can be refreshed without disturbing the physics.