trapped in an optical lattice—a series of pancake-shaped wells made of laser light. Another laser that "ticks" 518 trillion times per second provokes a transition between two energy levels in the atoms. The large number of atoms is key to the clocks' high stability. Visible light waves oscillate faster than microwaves, and therefore optical clocks can be more precise than caesium atomic clocks. The Physikalisch-Technische Bundesanstalt is working on several such optical clocks. The model with one single ytterbium ion caught in an ion trap is highly accurate. The optical clock based on it is exact to 17 digits after the decimal point. A pair of experimental atomic clocks based on ytterbium atoms at the National Institute of Standards and Technology has set a record for stability. NIST physicists reported in the August 22, 2013 issue of Science Express that the ytterbium clocks' ticks are stable to within less than two parts in 1 quintillion (1 followed by 18 zeros), roughly 10 times better than the previous best published results for other atomic clocks. The clocks would be accurate within a second for a period comparable to the age of the universe. Doping of stainless steel Ytterbium can also be used as a dopant to help improve the grain refinement, strength, and other mechanical properties of stainless steel. Some ytterbium alloys have rarely been used in dentistry. Ytterbium as dopant of active media The Yb3+ ion is used as a doping material in active laser media, specifically in solid state lasers and double clad fiber lasers. Ytterbium lasers are highly efficient, have long lifetimes and can generate short pulses; ytterbium can also easily be incorporated into the material used to make the laser. Ytterbium lasers commonly radiate in the 1.06–1.12 µm band being optically pumped at wavelength 900 nm–1 µm, dependently on the host and application. The small quantum defect makes ytterbium a prospective dopant for efficient lasers and power scaling. The kinetic of excitations in ytterbium-doped materials is simple and can be described within the concept of effective cross-sections; for most ytterbium-doped laser materials (as for many other optically pumped gain media), the McCumber relation holds, although the application to the ytterbium-doped composite materials was under discussion. Usually, low concentrations of ytterbium are used. At high concentrations, the ytterbium-doped materials show photodarkening (glass fibers) or even a switch to broadband emission (crystals and ceramics) instead of efficient laser action.