Direct Laser Cooling Al^+ Ion Optical Clocks

The Al^+ ion optical clock is a very promising optical frequency standard candidate due to its extremely small black-body radiation shift. It has been successfully demonstrated with the indirect cooled, quantum-logic-based spectroscopy technique. Its accuracy is limited by second-order Doppler shift, and its stability is limited by the number of ions that can be probed in quantum logic processing. We propose a direct laser cooling scheme of Al^+ ion optical clocks where both the stability and accuracy of the clocks are greatly improved. In the proposed scheme, two Al^+ traps are utilized. The first trap is used to trap a large number of Al^+ ions to improve the stability of the clock laser, while the second trap is used to trap a single Al^+ ion to provide the ultimate accuracy. Both traps are cooled with a continuous wave 167 nm laser. The expected clock laser stability can reach 9.0\times10^-17/\sqrt\tau. For the second trap, in addition to 167 nm laser Doppler cooling, a second stage pulsed 234 nm two-photon cooling laser is utilized to further improve the accuracy of the clock laser. The total systematic uncertainty can be reduced to about 1\times10^-18. The proposed Al^+ ion optical clock has the potential to become the most accurate and stable optical clock.