(color online) Schematic diagram of the feedback set-up. The optimal geometrical overlap of optical trap with a radius of 2.3 μm with an MOT trapping volume can be achieved by light shift of trapped atoms. Light induced fluorescence (LIF) photons of trapped atoms are observed with an avalanche photodiode (APD) in single photon counting mode with high signal-to-noise ratio (SNR). Part of the signal is then fed to a control system which discriminates the different characteristic states of trap. Ultra-violet light is employed to control the loading rate of trap in arange of 1–10⁵. Blue detuned laser is used to achieve the controlled collision of trapped atoms in which only one of the collision partners is lost. The MCS is the multi-channel scaler for analyzing the fluorescence of the atoms in the trap.