  Energy-efficient classical information processing and storage based on
topological defects in magnetic systems have been studied over past decade. In
this work, we introduce a class of macroscopic quantum devices in which a
quantum state is stored in a topological defect of a magnetic insulator. We
propose non-invasive methods to coherently control and readout the quantum
state using ac magnetic fields and magnetic force microscopy, respectively.
This macroscopic quantum spintronic device realizes the magnetic analog of the
three-level rf-SQUID qubit and is built fully out of electrical insulators with
no mobile electrons, thus eliminating decoherence due to the coupling of the
quantum variable to an electronic continuum and energy dissipation due to Joule
heating. For a domain wall sizes of $10-100$~nm and reasonable material
parameters, we estimate qubit operating temperatures in the range of $0.1-1$~K,
a decoherence time of about $0.01-1$~$\mu$s, and the number of Rabi flops
within the coherence time scale in the range of $10^{2}-10^{4}$.
