I just read a review paper on Boson-Einstein condensation for the first time. I have 2 questions. 1.Can anyone tell me why all experimental realization of BEC are done in alkali metal gas, like Na, Cs? Can the interaction between atoms be totally neglected? 2.A facinating property of BEC gas is its coherence, i.e. all atoms are precisely in the same state(ground state) with the same phase factor. I don't know why. Suppose as the tempreture is lowered, the atoms go to the ground state one by one, why they will neccessarily be in the same phase?