Abstract

For the magnetic trapping and evaporative cooling techniques recently used to obtain Bose Einstein Condensation (BEC) [1-3], it is necessary to load a large number of atoms into a trap with a long lifetime. Loading a magnetic trap from a Magneto-Optical Trap (MOT) often achieves one or other of these criteria, but rarely both. For caesium, typical results range from approximately 1×10^s atoms with a 1/e trap lifetime of 1 second, to 1×10⁵ atoms with a lifetime of over 200 seconds. The difficulty at very low pressure is that the MOT is limited by light assisted collisional loss, however the method of dark Magneto-Optical Trapping [4] (otherwise called the dark Spontaneous force Optical Trap, or dark SPOT) effectively eliminates this process, so that the predominant loss mechanism is collisions with room-temperature background atoms. This means that with a low background pressure, the dark MOT technique improves both the number and lifetime of atoms in the trap. Recently Anderson et al [5] used a dark MOT with rubidium and trapped 5×10⁷ atoms with a lifetime of 240 seconds, thus achieving more than an order of magnitude improvement over the MOT, but such dramatic improvements with caesium have not been reported.

© 1996 Optical Society of America