Multifrequency evaporative cooling to Bose-Einstein condensation in a high magnetic field

We demonstrate a way to circumvent the interruption of evaporative cooling observed at high bias field for Rb-87 atoms trapped in the (F = 2,m = +2) ground state. Our scheme uses a three-frequency rf knife achieved by mixing two rf frequencies. This compensates part of the nonlinearity of the Zeeman effect, allowing us to achieve Bose-Einstein condensation where the standard one-frequency rf knife evaporation method did not work We are able to get efficient evaporative cooling, provided that the residual detuning between the transition and the rf frequencies is smaller than the power broadening of the rf transitions at the end of the evaporation ramp.