We discuss methods of generation of quantum entangled pairs of photons and present results obtained by non-linear process of four way mixing (FWM) in alkali atoms. Two-mode squeezing, known also as relative amplitude squeezing, by a pair of photons generated by FWM has been thoroughly investigated because of important applications of such paired photons. We show strong intensity squeezing in potassium vapour and also dependence of IDS on FWM parameters. Experimental results are compared with calculated IDS by the model of operators. Examples of application of twin photons for quantum enhanced sensing and quantum imaging and microscopy will be given.