![]() Measurements of the clocks are also supplied to the International Bureau of Weights and Measures (BIPM) for use in the calculation of UTC. Together, these clocks form a highly resilient system that ensures uninterrupted operation of the time scale. UTC(NPL) is based on continuously-running commercial atomic clocks of two complementary types: active hydrogen masers and caesium clocks. The NPL time scale is termed UTC(NPL), and provides the reference for precise timekeeping in the UK. However, UTC is computed monthly and is not available directly it is instead accessed through the time scales maintained by the contributing institutes. All precise timekeeping and frequency measurement worldwide is based on UTC. Timekeeping today is coordinated globally, with around 70 national timing institutes contributing to the generation of the international reference time scale, Coordinated Universal Time (UTC). The MSF radio time signal and NTP-based internet time service are widely used to synchronise clocks and time-stamping equipment, while the NPL Time ® service provides an accurate, managed time service over optical fibre links to the finance sector. By applying very fine adjustments to the clock frequencies, UTC(NPL) is kept within a few nanoseconds of UTC.Īs the UK's centre for precise time and frequency measurement, NPL disseminates reliable and trusted time to industrial, business and public users across the UK through a range of services. Together, these clocks provide the reference time scale for the UK and contribute to the generation of the global reference time system, Coordinated Universal Time (UTC). The duration of the time scale unit interval is fine-tuned by two caesium fountain primary frequency standards, capable of determining a deviation in UTC(NPL) of one part in 10 15 (or 100 picoseconds) over one day. ![]() Complex laboratory systems measure the second with the highest achievable accuracy, while other clocks, able to run without interruption over many years, provide the essential continuity needed for keeping time.Īt NPL, a group of commercial atomic clocks act as the continuous 'flywheel' for the national time scale, UTC(NPL). The modern world relies on precise timing, based on highly stable atomic clocks. What you want, as with microwave clocks, is a "narrow" transition, which means it should have a long lifetime.We maintain the national time scale and the primary standards for frequency, contribute to global timekeeping, and disseminate accurate time and frequency to users across the UK. These days, it has become possible to use optical transitions at much higher frequencies (and therefore higher Q's), and there is a lot of work on "optical" clocks. Another advantage Cs has is that there is only one stable isotope, meaning you don't have any issues arising from having multiple isotopes around, or having to purify it. Other atoms and ions have been in common use, including Rubidium, Hydrogen, and Mercury ions. Because of the technology available at the time, microwave transitions such as this (9192631770 Hz for Cs) were the limit of what could be measured. Put another way, as an oscillator, it has a very high Q. That means for a given interrogation time, you will maximize the number of oscillations between these two states, giving a more precise measurement. Cesium has the advantage of having the largest hyperfine structure, that is, the energy difference of the two electron spin states in the presence on the nucleus's magnetic field. Since they have a single electron in addition to a filled shell, they have a fairly simple electronic structure. ![]() As sophiecentaur has noted, there are reasons for using alkali atoms, the ones on the left-hand side of the periodic table. I don't know if today is possible to use other systems instead of caesiumĬesium was actually chosen for a number of reasons. Then a counter detect the exact frequency of the oscillator, so stabilized, that is the number of cycles every second, or the second, knowing the number of cycles so counted. This variation is measured by a photodetector and immediately the electronic circuit corrects the oscillator's frequency to bring again the light intensity to its maximum value. When the exciting frequency varies slightly, the atoms, in those experimental conditions, cannot absorb that frequency anymore and so the emitted light intensity is reduced. ![]() Remember that, for example, caesium was one of the first metals to be used for photoelectric cells because you can ionize it with visible light. Caesium was initially used because its electronic levels can be excited with radiofrequencies (microwaves) produced by an electronic circuit and then they de-excitate generating light (don't know the details of the process) you can't do the same, in a rather simple way, with other atoms. ![]()
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