Kelvin-Temperature Ultracold Electron Beam

We report an ultracold electron beam with Kelvin-level transverse temperature generated from a cold atom electron source by two-color near-threshold photoionization of 87 Rb atoms in a magneto-optical trap. A novel self-referenced scheme was employed to precisely determine its transverse temperature. In this scheme, a 3×3 beam array was generated to provide an in situ reference, allowing the transfer-matrix element to be determined directly from the measured grid pattern. The transverse momentum spread of the central beam at the source plane is then retrieved, yielding a transverse temperature of T = 2.0 ± 1.0 K, with the corresponding intrinsic coherence length of L c = 21.1 ±5.0 nm and a relative coherence length of C ⊥ = (1.3 ± 0.4) × 10 -3 . The new scheme significantly suppresses the influence of residual magnetic fields, electric field inhomogeneities and electron-optical aberrations. A non-monotonic dependence of the transverse temperature on excess energy was observed and was qualitatively explained by the simulation using a polarization-dependent single-atom Coulomb-Stark trajectory model.