![]() Synchronisation specialist Chronos Technology Ltd has integrated a chip-scale atomic clock with a GPS receiver to prolong the time during which the clock is synchronized with the satellite signals c Chronos Technology LimitedĪ quiet revolution is also taking place in the area of ‘secondary’ industrial clocks and time standards. Researchers are now developing a new type of atomic clock based on optical atomic transitions that will allow time to be measured with a stability corresponding to one second in the age of the universe. The first atomic clock based on microwave atomic transitions was accurate to around one second in 300 years, making it 10 times more accurate than quartz-based clocks. Lord Kelvin first introduced the idea of using atomic transitions to measure time in 1879, but it was not until the 1950s that the concept was practically demonstrated. By the 20th century, more accurate clocks regulated by the steady vibration of quartz crystals began to replace mechanical devices. The problem was solved by John Harrison’s revolutionary H3 sea clock, which removed the need for a pendulum. Clocks in those days tended to lose or gain time and did not operate reliably at sea or in different climatic conditions. It was during this period that horologists first suggested that a precision clock might hold the key to the longitude problem by allowing mariners to accurately transfer ‘port time’ to sea. Timekeeping based on mechanical systems such as pendulums started to gain popularity in the 16th century. This led to the internationally agreed definition of a second being based on atomic time © National Physics Labaratory. Calibration of the caesium standard atomic clock was carried out by the use of the astronomical time scale ephemeris time (ET). ![]() The first accurate atomic clock, a caesium standard based on a certain transition of the caesium-133 atom, was built by Louis Essen (pictured right) in 1955 at the National Physical Laboratory (NPL) in Middlesex. This led to the internationally agreed definition of a second being based on atomic time © National Physical Laboratory ![]() These clocks, which work by launching a cloud of ultra-cold caesium atoms upwards and interrogating them using microwave radiation, require a vacuum vessel that stands nearly 3m tall in addition to extensive electronics racks and optoelectronic components to cool the atoms to micro-Kelvin temperatures. The state of the art in atomic timekeeping is being driven forward by institutions such as the NPL and similar labs such as the National Institute for Standards and Technology (NIST) in Colorado, US, with the latest atomic fountains offering stabilities of one second in more than 100 million years. The beat of Essen’s atomic clock was provided by microwave radiation whose frequency is adjusted to coincide with an atomic absorption, thereby exciting electrons in the atoms to move between particular discrete energy levels. This was the first SI unit to be defined in terms of a fundamental unchanging quantity, paving the way for more robust definitions of the metre and other units. Since 1967, atomic clocks have defined our base unit of time, the second, as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. When physicist Louis Essen demonstrated the world’s first accurate atomic clock in 1955 at the National Physical Laboratory (NPL) in the UK, it was not possible to foresee the impact that it would have on society. ![]() Global satellite navigation systems, the internet and utility infrastructures all rely on the accurate timing signals provided by atomic clocks, with transport infrastructures and logistics, safety-of-life critical systems and even global financial transactions increasingly demanding better timing accuracy. Our ability to determine time with ever-increasing precision underpins modern life. Dr John Christensen and Professor Patrick Gill of the Time and Frequency Group at the UK’s National Physical Laboratory and Sir Peter Knight of Imperial College London and past President of the Institute of Physics, describe a new generation of miniature atomic clocks that promise the next revolution in timekeeping. Over the last five decades, the passage of time has been defined by room-sized atomic clocks that are now stable to one second in 100 million years. Strontium ion optical clock © National Physical Laboratory
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