Quantum squeezing enhancement based on phase-sensitive cascaded four-wave mixing processes

Quantum squeezing is an important quantum resource for quantum metrology as it can improve the measurement precision. By enhancing the quantum squeezing level, the measurement precision can be further improved. Here, we experimentally implement quantum squeezing enhancement based on phase-sensitive cascaded four-wave mixing (CFWM) processes. The intensity-difference squeezing (IDS) between the two outputs of the phase-sensitive CFWM processes is enhanced to about 7.42 dB compared with IDS generated by the single four-wave mixing (FWM) process (about 3.31 or 4.01 dB). Such enhancement is enabled by both the intrinsic interference nature of phase-sensitive CFWM processes and the contribution of more gain from the two FWM processes. In addition, we measure IDS levels generated by phase-sensitive CFWM processes at different internal-phase-locking points, which shows that the internal phase plays an important role in IDS enhancement. Our scheme provides a new method for improving the degree of IDS and may find potential applications in enhancing the measurement precision in quantum metrology.