Experimental generation of quadruple quantum-correlated beams from hot rubidium vapor by cascaded four-wave mixing using spatial multiplexing

Multimode quantum states, such as multipartite quantum entanglement or quantum correlations, are important for both fundamental science and the future development of quantum technologies. Here we theoretically propose and experimentally realize a scheme that can fully exploit the multi-spatial-mode nature of the four-wave-mixing (FWM) process, i.e., spatial multiplexing, and thus integrates multiple FWM processes into a single cell at each stage of the cascaded process. The number of generated quantum-correlated beams 2(n) is exponentially dependent on the number of vapor cells n. In addition, the quantum correlations between the multiple beams also increase as the number of vapor cell increases. For the case of n = 2, we experimentally show that the degree of intensity-difference squeezing between the four quantum-correlated beams in our scheme is enhanced to -8.2 +/- 0.2 dB from -5.6 +/- 0.3 and -6.5 +/- 0.2 dB of squeezing obtained with a single FWM process. Our system may find applications in quantum information and precision measurement.