  Reactive power plays a crucial role in the design of small antenna systems,
but its impact on the bandwidth of quantum emitters is typically disregarded.
Here, we theoretically demonstrate that there is an intermediate domain between
the usual weak and strong coupling regimes where the bandwidth of a quantum
emitter is directly related to the dispersion properties of the reactive power.
This result emphasizes that reactive power must be understood as an additional
degree of freedom in engineering the bandwidth of quantum emitters. We
illustrate the applicability of this concept by revisiting typical
configurations of quantum emitters coupled to resonant cavities and waveguides.
Analysis of the reactive power in these system unveils new functionalities,
including the design of effcient but narrowband photon sources, as well as
quantum emitters exhibiting a bandwidth narrower than its nonradiative
linewidth.
