  Collective excitation modes in solid state systems play a central role in
circuit quantum electrodynamics, cavity optomechanics, and quantum magnonics.
In the latter, quanta of collective excitation modes in a ferromagnet, called
magnons, interact with qubits to provide the nonlinearity necessary to access
quantum phenomena in magnonics. A key ingredient for future quantum magnonics
systems is the ability to probe magnon states. Here we observe individual
magnons in a millimeter-sized ferromagnet coherently coupled to a
superconducting qubit. Specifically, we resolve magnon number states in
spectroscopic measurements of a transmon qubit with the hybrid system in the
strong dispersive regime. This enables us to detect a change in the magnetic
dipole of the ferromagnet equivalent to a single spin flipped among more than
$10^{19}$ spins. The strong dispersive regime of quantum magnonics opens up the
possibility of encoding superconducting qubits into non-classical magnon
states, potentially providing a coherent interface between a superconducting
quantum processor and optical photons.
