| Summary: | Squeezed vacuum states at audio-band frequencies are important quantum resources for practical applications. We demonstrated the generation of squeezed vacuum states at the audio-band frequencies from a subthreshold optical parametric oscillator with a periodically poled KTiOPO<sub>4</sub> crystal pumped by a homemade continuous wave single-frequency dual-wavelength laser. To detect squeezed vacuum states at audio-band frequencies, the influences of the local oscillator (LO) power, the common mode rejection ratio (CMRR) of balanced homodyne detectors, and the phase jitter between the LO and squeezed vacuum field on the measurement of squeezed vacuum states at audio-band frequencies were considered. By optimizing the LO power, improving the CMRR of photodetectors to 67 dB based on the design of differential fine-tuning circuit and adjustable bias voltage, and reducing the phase jitter between the LO and squeezed vacuum field to 1.7° with the help of the coherent locking technique, 6.1 ± 0.3 dB squeezed vacuum states at audio frequencies from 5 kHz to 20 kHz were generated. A 3.0 ± 0.3 dB phase squeezed vacuum state was obtained at the audio frequency of 3.5 kHz.
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