Enabling Chip-Based Atom Interferometry 1: Long Coherence Time and Number Squeezing with $^{23}$Na BECs

Using a combination of DC and RF magnetic fields, we have coherently split a BEC into two separated BECs on an atom chip. We find that the split BECs maintain a relative phase coherently for up to 200 ms, which we read out using absorption imaging of matter wave interference after time-of-flight expansion. This is a factor of 10 longer than the phase diffusion time for a coherent state at our atom number and density. We attribute the long coherence time to number squeezing by a factor of 10 caused by mean-field interactions during the splitting process, which reduces the phase diffusion rate. In spite of the presence of strong atom-atom interactions, the system potentially allows us to implement a BEC interferometer on an atom chip. G. --B. Jo et al., PRL 98, 030407 (2007)