Tunable single-photon diode and circulator via chiral waveguide–emitter couplings

We propose a scheme to realize a single-photon diode and circulator using two waveguides chirally coupled to a five-level M-type atom. Two external control fields are introduced to drive the emitter. Non-reciprocal single-photon propagation can be completed in our scheme, which underpins the single-photon diode and circulator. The single-photon diode can work well at special frequency points of the incident photon simultaneously. We can modulate the Rabi frequencies and detunings of the control fields to satisfy various values of frequency points of the incident photon. In addition, the non-reciprocity of the photon propagation can be turned off or on when the control fields are turned off or on. A single-frequency filter can filter out a special frequency for the incident photon by adjusting the detunings of the control fields. The single-photon circulator can realize photon propagation in the pathways $1\to 2,2\to 3,3\to 4,4\to 1$ with probability 100%. The properties of the circulator can be modulated by adjusting the Rabi frequencies and detunings of the control fields. (letter)