Strontium Atoms "See" Blue Light
"1156 stainless steel micro-tubes (0.1mm ID, 5mm long) are clamped in a 2cm triangular nozzle to channel strontium atoms into a 50cm long Zeeman slower. In the high-vacuum Â (Knudsen) flow regime, the nozzle acts as a brick wall to atoms not traveling in the axial direction, filtering out atoms with substantial transverse velocity. Traveling through the micro-tubes and the Zeeman slower, theÂ atoms absorb counter-propagating resonant 461nm (blue) photons, which transfer enough momentum to slow the atoms from nearly a kilometer per second to a few metersÂ per second. The Zeeman slower's strongly varying magnetic field keeps the color of the light blue in the atoms' frame of reference; without it the atoms would "see" the laser's colorÂ gradually shift towards green, yellow and red. The perceived change in color is due to the Doppler effect, and is analogous to the apparentÂ down-shift in pitch from a receding sound source.