Phase modulation spectroscopy: a non-destructive probe of Bose-Einstein condensates

The measurement of the optical phase of far-detuned laser light by phase mo dulation spectroscopy is promising as a non-destructive probe of the dynami cs of Bose-Einstein condensates. We have developed the experimental techniq ue on a room-temperature Cs vapour cell, and a Cs magneto-optical trap (MOT ). We measure column densities with an accuracy better than 7%. A numerical model, including Doppler broadening and transitions between all relevant h yperfine levels, accurately describes the data from the Cs cell. The same m odel without Doppler broadening gives quantitative agreement with measureme nts from the Cs RIOT. We show that the conditions needed for non-destructiv e measurements of BEC dynamics are achievable with conventional equipment a nd a simple, robust experimental setup.