Strontium Ion Optical Frequency Standard
The strontium ion optical frequency standard is based on the electric quadrupole transition at 674 nm in 88Sr+, which has a natural linewidth of 0.4 Hz. As a result of absolute frequency measurements carried out at NPL in 2004, together with work at the National Research Council in Canada, it is one of the first optical frequency standards to be recommended as secondary representations of the second. The standard shows excellent prospects for the development of an optical clock whose performance exceeds that of the current primary caesium standard.
A partial term diagram of 88Sr+, showing the levels relevant to the operation of the optical frequency standard, is shown above. The ion is confined in an electromagnetic trap, which produces an oscillating quadrupole potential. In order to laser cool the ion, radiation is required at both 422 nm and 1092 nm. The probe laser system, which provides the optical frequency standard, drives the 5s2S1/2 - 4d2D5/2 electric quadrupole transition at 674 nm. When the ion is in the D5/2 level, it is rapidly returned to the cooling cycle using a 1033 nm laser tuned to the 2D5/2 - 2P3/2 transition. The overall experimental arrangement is shown below. One of the attractions of this system is that the laser wavelengths required are all produced using diode laser based systems. The natural width of the electric quadrupole clock transition is 0.4 Hz, limited by the 2D5/2 state lifetime.
The 422 nm cooling radiation, which drives the 5s2S1/2 - 5p2P1/2 transition, is provided by a frequency doubled 844 nm diode laser. When the ion is in the 2P1/2 state, it can decay to the long-lived 4d2D3/2 state with a 1/13 probability and so a diode laser is required to drive the 1092 nm 2D3/2 - 2P1/2 transition, returning the ion to the cooling cycle. In a low magnetic field (typically 1 µT) this laser would normally drive the ion into dark states of the 2D3/2 - 2P1/2 transition, and the ion would not fluoresce and be cooled. The actual dark state depends upon the polarization of the laser. However, by modulating the polarization at a few MHz, the dark state is removed and fluorescence is restored.
The laser at 674 nm provides the optical frequency standard that drives the 2S1/2 - 2D5/2 transition. This laser ideally needs to have a linewidth comparable to that of the natural linewidth of the transition, which is 0.4 Hz. Currently, at NPL, we have demonstrated a linewidth of <2 Hz on a 3 s timescale, broadening to ~4 Hz at 30 s. In order to achieve this narrow linewidth, the laser has to be locked to a very stable and ultra-low-expansion (ULE) high finesse cavity. The ULE cavity is not tunable and so the frequency difference between the nearest cavity mode and the strontium transition frequency needs to be bridged by an acousto-optic modulator (AOM).