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We describe how a coherent optical drive that is near-resonant with the upperrungs of a three-level ladder system, in conjunction with a short pulseexcitation, can be used to provide a frequency-tunable source of on-demandsingle photons. Using an intuitive master equation model, we identify twodistinct regimes of device operation: (i) for a resonant drive, the sourceoperates using the Autler-Townes effect, and (ii) for an off-resonant drive,the source exploits the ac Stark effect. The former regime allows for a largefrequency tuning range but coherence suffers from timing jitter effects, whilethe latter allows for high indistinguishability and efficiency, but with arestricted tuning bandwidth due to high required drive strengths and detunings.We show how both these negative effects can be mitigated by using an opticalcavity to increase the collection rate of the desired photons. We apply ourgeneral theory to semiconductor quantum dots, which have proven to be excellentsingle-photon sources, and find that scattering of acoustic phonons leads toexcitation-induced dephasing and increased population of the higher energylevel which limits the bandwidth of frequency tuning achievable while retaininghigh indistinguishability. Despite this, for realistic cavity and quantum dotparameters, indistinguishabilities of over $90\%$ are achievable for energyshifts of up to hundreds of $\mu$eV, and near-unity indistinguishabilities forenergy shifts up to tens of $\mu$eV. Additionally, we clarify theoften-overlooked differences between an idealized Hong-Ou-Mandel two-photoninterference experiment and its usual implementation with an unbalancedMach-Zehnder interferometer, pointing out the subtle differences in thesingle-photon visibility associated with these different setups.

Using the Autler-Townes and ac Stark effects to optically tune the frequency of indistinguishable single-photons from an on-demand source