Fig. 2

Optical collider procedure. a ⁴⁰K (blue, right) and ⁸⁷Rb (red, left) atoms, prepared in the desired internal quantum states, are held in two crossed optical dipole traps separated by ~3 mm; COM indicates the centre-of-mass for pairs of K and Rb atoms. b The two traps are accelerated towards each other, keeping m_Rbv_Rb = −m_Kv_K, so that pairs of of K and Rb atoms have zero total momentum on average, and their COM is at rest in the laboratory frame. c When the wells are separated by ≈60 μm, the optical traps are switched off. d The two clouds collide in free space. e, f The K and Rb collision halos expand at different rates. We image the K halo at the time represented in e, and then wait to image Rb until its halo has expanded to the equivalent size (f). g, h 3D collision halos of K (g) and Rb (h) are projected onto a 2D plane by the process of absorption imaging. We integrate the projected images in the vertical direction to obtain line densities. Line densities are fitted to extract the scattered fraction