MACROSCOPIC QUANTUM TUNNELING OF A BOSE CONDENSATE

We study, by means of a variational method, the stability of a condens ate in a magnetically trapped atomic Bose gas with a negative scatteri ng length and find that the condensate is unstable in general. However , for temperatures sufficiently close to the critical temperature the condensate turns out to be metastable. For that case we determine in t he usual WKB approximation the decay rate of the condensate due to mac roscopic quantum fluctuations. When appropriate, we also calculate the decay rate due to thermal fluctuations. An important feature of our a pproach is that (nonsingular) phase fluctuations of the condensate are taken into account exactly.