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

The measurement of the optical phase of far-detuned laser light by phase modulation spectroscopy is promising as a non-destructive probe of the dynamics of Bose-Einstein condensates. We have developed the experimental technique 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 hyperfine levels, accurately describes the data from the Cs cell. The same model without Doppler broadening gives quantitative agreement with measurements from the Cs MOT. We show that the conditions needed for non-destructive measurements of BEC dynamics are achievable with conventional equipment and a simple, robust experimental setup.