  Quantum jumps of a qubit are usually observed between its energy eigenstates,
also known as its longitudinal pseudo-spin component. Is it possible, instead,
to observe quantum jumps between the transverse superpositions of these
eigenstates? We answer positively by presenting the first continuous quantum
nondemolition measurement of the transverse component of an individual qubit.
In a circuit QED system irradiated by two pump tones, we engineer an effective
Hamiltonian whose eigenstates are the transverse qubit states, and a dispersive
measurement of the corresponding operator. Such transverse component
measurements are a useful tool in the driven-dissipative operation engineering
toolbox, which is central to quantum simulation and quantum error correction.
