A prototype differential atom interferometer for fundamental physics

Cooling sequence The cold-atom apparatus used in this experiment has previously been described in refs. 42 , 56 . To prepare samples of cold 87 Sr, the atoms are first collected over 1.5 s in a blue three-dimensional MOT that uses the 1 S 0 → 1 P 1 transition at 461 nm and a field gradient of 3.5 mT cm −1 . Atoms that leak into the metastable 3 P 2 manifold are recycled into the MOT using repump lasers at 679 nm and 707 nm. For efficient repumping of 87 Sr, frequency sidebands at 585 MHz and 487 MHz are applied to the 707-nm light using an electro-optic modulator to create frequency components near-resonant with transitions from all five hyperfine manifolds of 3 P 2 (ref. 57 ). When the blue MOT is switched off, the atoms are captured in a red MOT operating on the 1 S 0 F = 9/2 to \({}^{3}P_{{1}}\,{F}^{{\prime} }=11/2\) transition at 689 nm, using a field gradient of 390 μT cm −1 . Sidebands at 1,463.265 MHz are applied to the 689-nm light using a resonant electro-optic modulator, such that the F = 9/2 to \({F}^{{\prime} }=9/2\) transition stirs the atoms between Zeeman sublevels of the ground state, thus mitigating losses into sublevels where atoms are weakly confined 28 . During the first 220 ms in the red MOT, an intensity of 1,800 I sat is used for each of the six MOT beams, where I sat = 3 μW cm −2 is the saturation intensity of the 689-nm transition. …