LAUSR

We study single-photon scattering at a giant Λ-type atom, where both atomic transitions are coupled with the modes of a single waveguide at two separated points. The giant-atom structure brings phase-dependent interference effects to both elastic (frequency-preserving) and inelastic (frequency-converting) scattering process, and thus leads to a series of interesting phenomena, such as perfect transmission over the whole frequency range and total reflection. The condition of the optimal frequency conversion is also identified and shown to be phase-dependent. We also consider the combination of the giant-atom interference and the Sagnac quantum interference by further inserting a Sagnac interferometer at each of the coupling points. It shows that the two kinds of interference effects are compatible and play their roles independently, such that efficient frequency conversion with unit efficiency can be achieved in this case in addition to all the aforementioned special phenomena.