Ultracold strontium atoms enable super-accurate optical lattice clock

The trapping of laser-cooled strontium (Sr) atoms may enable optical clocks that are more accurate and stable than the current cesium atomic clock, say physicists at Physikalisch-Technishe Bundesanstalt (PTB; Braunschweig, Germany). Such optical clocks use the oscillation of light waves as their “pendulum,” rather than the microwave oscillations of cesium devices. The 88Sr atoms are first Doppler cooled to 2 mK via the broad 1S0–1P1 transition at 461 nm. The second cooling step involves the spin-forbidden 1S0–3P1 transition at 689 nm, which minimizes reabsorption of spontaneous emission. To reach temperatures around 1 µK, 28 ms of single-frequency cooling follows 50 ms of broadband cooling, with a transfer efficiency of 23%, leaving a cloud of atoms in a far-off resonance optical lattice trap formed by the interference pattern of two 813 nm laser beams. Simultaneous operation of a magneto-optical trap and lattice trap enable a transfer efficiency of atoms into the dipole trap of approximately 60%. The high atomic density will enable the precise evaluation of frequency shifts that might eventually lead to an improved definition of the second. Contact Uwe Sterr at [email protected].