Fig. 4
From: Quantum wave mixing and visualisation of coherent and superposed photonic states in a waveguide
Quantum wave mixing with two-photon superposed states and sensing of quantum states. The mixing spectrum with the three-level atom consists of five peaks, which is a result of the mapping of two-photon state |γ〉− (N ph = 2). Comparing with Fig. 3, an additional emission peak at 3ω + − 2ω − appears, corresponding to two-photon emission from |γ〉−. The absorption process resulting in a peak at 2ω − − ω + is now possible, as it is schematically exemplified. Importantly, the device probes the photonic states of the coherent field, distinguishing classical (Fig. 2a) (N ph = ∞), one- (N ph = 1), and two-photon (N ph = 2) superposed states
