Figure 1
From: Controllable microwave three-wave mixing via a single three-level superconducting quantum circuit
(a) Schematic diagram for a SFQC with three Josephson junctions biased by a magnetic flux Φe and also driven by the magnetic flux 
with different frequencies ωl, which are specified in panels (b,c); EJ is the Josephson energy and 0.5 < α < 1. (b) A three-level (qutrit) SFQC, which can be considered as an artificial atom with Δ-type (cyclic) transitions driven by the external magnetic flux Φ(ω1) [Φ(ω2)] with frequency ω1 (ω2) to induce the transition between the energy levels |1〉 and |2〉 (|2〉 and |3〉), results in the generation of the output signal with the sum-frequency ω+; (c) Same as in panel (b) but for the flux Φ(ω1) inducing the transition between the energy levels |1〉 and |3〉, leads to the generation of the output signal with the difference-frequency ω−.
