Multi - frequency evaporative cooling to BEC in a high magnetic field

We demonstrate a way to circumvent the interruption of evaporative cooling observed at high bias field for 87 Rb atoms trapped in the (F = 2, m = +2) ground state. Our scheme uses a 3-frequencies-RF-knife achieved by mixing two RF frequencies. This compensates part of the non linearity of the Zee-man effect, allowing us to achieve BEC where standard 1-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. Forced evaporative cooling of atoms [1,2] in a magnetic trap is at the moment the only known way to achieve Bose-Einstein condensation [3–5]. Particles with energy significantly larger than the average thermal energy are removed from the trap and the remaining ones thermalize to a lower temperature by elastic collisions. For that, a radiofrequency (RF) magnetic field is used to induce a multi-photon transition from a trapping state to a non-trapping state via all intermediate Zeeman sublevels. Atoms moving in the trap with sufficient energy can reach the resonance point (RF knife) and exit the trap. If the RF-frequency is decreased slowly enough, and no other process is hampering the forced-evaporation, the increase of the phase space density obtained by this method eventually leads to Bose-Einstein condensation. In a previous publication [6], we reported that RF forced evaporative cooling of 87 Rb atoms in the (F = 2, m = +2) ground state in a magnetic trap with a high bias field is hindered and eventually interrupted. Our interpretation of this phenomenon is based on the non-linear terms of the Zeeman effect that lift the degeneracy of transition frequencies between adjacent Zeeman sublevels. This interpretation is supported by numerical calculations [7]. Interrupted evaporative cooling in a large magnetic field is a serious problem in several situations, interesting for practical reasons-like the use of permanent magnets [8] or of an iron core electromagnet as the one described in [9]. High magnetic field evaporation is also important in connection with Feshbach resonances [10–13]. In this paper, we demonstrate that it is possible to achieve efficient evaporative cooling in a high magnetic field, by use of a multi-frequency RF knife allowing a multi-photon transition to take place across non equidistant levels. We show that, for our range of magnetic fields, …