Genuine quadripartite quantum steering generated by an optical parametric oscillation cascaded with a sum-frequency processS. L. Liang, Y. Liu, Y. B. Yu, J. Y. Lan, X. B. He and K. X. GuoEPL, 131 1 (2020) 10001DOI: https://doi.org/10.1209/0295-5075/131/10001