Fig. 1. (a) Optical frequency-shifting and interference property when an AOM is used as a bi-frequency beam splitter/combiner. (b) ABI whose output ports preserve the optical frequency at the output ports.

Fig. 2. Three typical functions fulfilled by the ABI. (a) Bi-frequency beam splitter with variable splitting ratio; (b) frequency tuner of quantum state; (c) optical switch controlled by gating the RF driving signal to have the AOMs chopped.

Fig. 3. (a) Experimental setup of the ABI. AOM, acousto-optic modulator; CM, concave mirror; PZT, piezo-electric translation device; PD, photodiode; SPD, single-photon detector; DAQ, data acquisition system. (b) 100 kHz beating signal detected by PD₁ from the light with microwatt level intensity; (c) 5 Hz beating signal detected with SPD from the light with single-photon level intensity. Red dashed-dotted lines in (b) and (c) are the fitting to the data.

Fig. 4. (a) Output intensity of an ABI without beating detected by PD₁ for PZT scanned (blue), locked to maximum (green), and locked to minimum (orange); (b) AOM driving signal RF₁ and RF₂ to realize phase dithering for the phase-locking scheme. Both RF₁ and RF₂ are centered at 80 MHz. The modulation for phase locking of the ABI is realized by adding a 200 kHz small signal phase modulation to RF₁. For illustration purposes, the upper trace is plotted with larger modulation strength and frequency than that used in the experiment.

Fig. 5. (a) Output intensity in switch mode detected by PD₁ with a locking duty cycle of 30%; in this mode, the AOM driving signals RF₁₍₂₎ are turned on and off periodically by a gate signal shown above the intensity result. (b) SPD counting result of the ABI in frequency tuner mode with a detection duty cycle of 50%. The error bar for counting results is the same size or smaller than the data point. The input light is a temporally complementary pattern of an LR light with intensity on the microwatt level and a weak Coh with intensity on the single-photon level. The timing sequences of LR and Coh are shown above the counting result. SPDe, SPD trigger-enabled signal.