$R85b$ tunable-interaction Bose–Einstein condensate machine

We describe our experimental setup for creating stable Bose–Einstein condensates (BECs) of $R85b$ with tunable interparticle interactions. We use sympathetic cooling with $R87b$ in two stages, initially in a tight Ioffe–Pritchard magnetic trap and subsequently in a weak, large-volume, crossed optical dipole trap, using the 155 G Feshbach resonance to manipulate the elastic and inelastic scattering properties of the $R85b$ atoms. Typical $R85b$ condensates contain $4×104$ atoms with a scattering length of $a=+200a0$⁠. Many aspects of the design presented here could be adapted to other dual-species BEC machines, including those involving degenerate Fermi–Bose mixtures. Our minimalist apparatus is well suited to experiments on dual-species and spinor Rb condensates, and has several simplifications over the $R85b$ BEC machine at JILA, which we discuss at the end of this article.