
 An ensemble of  $^{87}${Rb} atoms is trapped and cooled with a Magneto-Optical Trap (MOT).  To generate correlated photon pairs MOT is turned off and the atoms are excited to $5\text{D}_ {3/2},\,F=3$ level from the ground level $5\text{S}_{1/2},\,F=2$ using two orthogonal linearly polarised pump beams (795\,nm and 762\,nm).  The 795\,nm pump beam is red detuned by 30\,MHz from the intermediate level $5\text{P}_{1/2},\,F=2$ in order to minimise incoherent scattering back to the ground level.   
 From  the excited level, atoms decay back to the ground level via two intermediate hyperfine levels $5\text{P}_ {3/2},\,F=3$ or $5\text{P}_ {3/2},\,F=2$. Energy conservation and phase matching condition results in the generation of signal (776\,nm) and idler (780\,nm)  photon pairs. The correlations from the decay path $5\text{D}_{3/2},\,F=3,$ $\rightarrow$ $5\text{P}_{3/2},\,F=2$ $\rightarrow$ $5\text{S}_{1/2},\,F=2$  are suppressed  by projecting the generated photons onto certain polarisations []. The interference
filter(s) $F_{1}$  overlaps the two pump beams in co-propagating geometry inside the cloud and the $F_{2}$ separates the signal and idler photons from residual pump light. The signal photon detected by an Avalanche Photodetecter ($D_{t}$) heralds
the presence of single photon in idler mode [ ]
 The experimental sequence for the generation of correlated photon pairs comprises of 10 $\mu$s  period for the photon pair generation. This is interleaved with the periods of 80${\mu}$s when the MOT is turned on to replenish and cool the atomic cloud. 
 
 
 