Beating the 3 dB quantum squeezing enhancement limit of two-mode phase-sensitive amplifier by multi-beam interference

Quantum squeezing, which makes measurement sensitivity beyond classical limit by reducing system noise, is an essential non-classical resource for quantum metrology. It is of great importance to enhance quantum squeezing since the squeezing degree directly determines the extent to which measurement sensitivity beats the classical limit. Recently, a two-mode phase-sensitive amplifier has been utilized to enhance the quantum squeezing of phase-insensitive amplifier. However, such enhancement has an intrinsic limit of 3 dB. Here we show that such limit of 3 dB can be overcome by utilizing multi-beam interference. Specifically, a quantum squeezing enhancement of about 3.67 dB is observed by direct measurement. Moreover, we find that the amount of quantum squeezing enhancement increases as the number of multi-beam interference increases, which clearly shows that beating the quantum squeezing enhancement limit of 3 dB is induced by multi-beam interference. Our results here provide an efficient way to enhance the quantum squeezing.