Laser Cooled 9Be+ Accurate Clock

The use of laser cooled stored ions in an atomic frequency standard has the potential of very high accuracy because Doppler effects are greatly suppressed. A clock based on the ground-state hyperfine transition in ²⁰¹Hg has potential accuracy and stability exceeding 1 part in 10¹⁵. However, laser cooled ⁹Be ions are experimentally easier to obtain. Therefore a ⁹Be based frequency standard is investigated in order to study the generic problems of laser cooled stored ion frequency standards. Approximately 300 ⁹Be ions are stored in a Penning trap and laser cooled. The 303 MHz ground state (M_I, M_J)=(-3/2, 1/2) → (-1/2, 1/2)) nuclear spin flip hyperfine transition is observed at a magnetic field ~0.82T) where the transition frequency is independent of magnetic field to first order. The time domain Ramsey method of interrogation is used and yields a linewidth of 25 mHz. The stability of an oscillator locked to this transition has been measured for 400 s < τ < 3200 s to be σ_y(τ) ≃ 2x10¹¹ t^-1/2. By measuring the velocity distribution of the ions, the second-order Doppler shift is determined to be on the order of 5xl0^-14. The magnetic field instability contributes a 3x10^-14 uncertainty in the present experiment. All other systematic uncertainties are estimated to be no greater than 3xl0^-14.