Schematic of experimental setup. DBR: distributed Bragg reflector laser. PBS: polarizing beam splitter. BS: beam splitter. FEOM: fiber electro-optic modulator. AOM: acousto-optical modulator. SG: signal generator. Amp: microwave power amplifier. PD: photodetector. BPD: balanced photodetector. LNA: low-noise amplifier. λ / 2 and λ / 4 : half-wave and quarter-wave plates, respectively. TTL: transistor-transistor logic switch. PID: proportional-integral-derivative controller. FPGA: field-programmable gate array. BG: buffer gas. LO: local oscillator. SWPM: square-wave phase modulation. Mod.: lock-in amplifier modulation signal applied to the solenoid. GTP: Glan–Taylor polarizer. BG: buffer gas.