We present the experimental measurement of a squeezed vacuum state by means of a bichromatic local oscillator (BLO). A pair of local oscillators at ±5 MHz around the central frequency ω 0 of the fundamental field with equal power are generated by three acousto-optic modulators and phase-locked, which are used as a BLO. The squeezed vacuum light are detected by a phase-sensitive balanced-homodyne detection with a BLO. The baseband signal around ω 0 combined with a broad squeezed field can be detected with the sensitivity below the shot-noise limit, in which the baseband signal is shifted to the vicinity of 5 MHz (the half of the BLO separation into the vicinity of 5 MHz ((the half of the BLO separation) and sub-shot-noise detection is implemented. Thus this work with the BLO and broadband squeezing can be used to enhance the signal-to-noise ratio (SNR) of an interferometer for lower frequency phase measurement [24,25].The schematic diagram of the detection is shown in Fig. 1. A strong BLO (at ±Ω 0 around the central frequency ω 0 of the fundamental field with equal power) is mixed with the signal light field at a 50/50 beam splitter. The relative phase θ of the local oscillator and the signal field can be controlled by the reflective mirror mounted on a PZT (piezoelectric transducer). The annihilation operators of the bichromatic local oscillator and the signal field can be written aŝ a(t) =â + (t) exp[−i(ω 0 + Ω 0 )t] +â − (t) exp[−i(ω 0 − Ω 0 )t] andb(t) =b 0 (t) exp (−iω 0 t), whereâ +(−) (t) andb 0 (t) are the slow varying operators of the fields. The output 1
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