- National Physical Laboratory: Trapped ion optical frequency standards
- High-accuracy strontium ion optical clock; National Physical Laboratory (2005)
- National Research Council of Canada: Optical frequency standard based on a single trapped ion
- NIST: Definition of the second; notice the cesium atom must be in its ground state at 0 K
- Official BIPM definition of the second
- Seconds and leap seconds by the USNO
- The leap second: its history and possible future
- What is a Cesium atom clock?