  We propose a scheme to generate macroscopic Schr\"odinger-cat states in a
quantum harmonic oscillator (electromagnetic field or mechanical resonator)
coupled to a quantum bit (two-level system) via a conditional displacement
mechanism. By driving the qubit monochromatically, the oscillation of the qubit
state modifies the effective frequency of the driving force acting on the
oscillator, and a resonant or near-resonant driving on the oscillator can be
achieved. The displacement of the oscillator is then significantly enhanced due
to the small detuning of the driving force and can exceed that of the
zero-point fluctuation. This effect can be used to prepare quantum
superpositions of macroscopically distinct coherent states in the oscillator.
We present detailed studies on this state-generation scheme in both the closed-
and open-system cases. This approach can be implemented in various experimental
platforms, such as cavity- or circuit-QED systems, electromechanical systems,
and spin-cantilever systems.
