Nict (also spelled nikt) is a unit of measurement in the International System of Units (SI). It is defined as the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133 atom. The name “nict” is an acronym derived from “nitrogen clock”.
The SI unit for time, the second, is defined in terms of this radiation emission and reception. In 1967, when the International System was established, atomic clocks were not yet accurate enough to provide a practical realization of the second so that it was necessary to interpolate between pulses emitted by radio transmitters. The first such realization was provided by Louis Essen and Jack Parry working at National Physical Laboratory in Teddington, England who used a quartz crystal oscillator and an amplifier chain calibrated against a cesium beam atomic clock. This system became known as Caesium Fountain Atomic Clock or CSFAC. The pulse repetition rate of this early prototype caesium fountain was about once per day i.e. it would take about 24 hours for 9192631770 periods to elapse which made it impractical for general use but it did demonstrate that a caesium fountain could be built with sufficient accuracy to realize the second.
In 1971 Steve Jefferts working at NBS Boulder led a team that produced a more practical design which had a pulse repetition rate around 900 Hz giving one reading every 11 seconds or 86400 readings per day – still too slow for many applications but much better than before. Nevertheless, this system became operational in 1976 providing useful data on variations in atmospheric pressure and temperature during geomagnetic storms caused by coronal mass ejections from solar flares. A further improvement came in 1984 when Dick Early working at JPL led his team to produce an even higher performance device with 1 MHz output yielding 8640 000 readings per day – fast enough for many commercial applications including navigation where global positioning system (GPS) satellites use caesium beams as their primary timing reference. However, if even higher speeds are required other technologies such as rubidium atoms or optical lattices can be used instead.
The main advantage that nicts have over other units is their stability: they do not change over time like Quartz crystals which can drift due to changes in ambient temperature or pressure. For example, during World War II there were reports of German submarines using specially calibrated watches based on quartz crystals to help them surface at exactly the right moment so as not to be detected by Allies’ sonar equipment. If those watches had been based on nicts then they would have kept better time and hence improved chances of success for those missions. Even now, over 70 years later, modern versions of these nict-based watches are still available and prized by collectors due their high accuracy and durability under extreme conditions such as spaceflight where they have been used on all NASA manned missions since Apollo 7.