Telling time or retelling it?

The development of ultra accurate optical clocks will redefine several aspects in the measurement of time itself, writes Kalyan Ray

How accurate a clock can be? The time-keeping machines that we use in our daily lives, be it wristwatches or wall clocks, can go off by a few minutes every now and then. The clocks in the digital world are better, though they can go wrong over a longer period.

But can there be a clock that won’t miss a single second in 13.8 billion years – the age of the universe? Such precise time-keeping instruments are a reality now in the US, EU, Russia, Japan, Canada and China. Despite a late start, four Indian laboratories in three cities have now jumped onto the bandwagon to create clocks so ultra-precise that they will miss only one second in 300 billion years – over 20 times the age of the universe.

Known as optical clocks or quantum clocks, such time measurement gadgets will be an integral part of the futuristic world of quantum technologies. They will offer essential assistance to cutting-edge scientific experiments required to probe the fundamentals of the universe, such as gravitational wave detection, searching for evidence of dark matter and dark energy, testing Einstein’s general theory of relativity and many more.

Among other things, they can also be used to accurately map the geode, which will have serious strategic applications. This includes early warnings of earthquakes, based on minute tectonic plate displacements. Soon, they will also play the central role in redefining the international definition of “second” — the basic unit of time which, at present, is based on Caesium clocks.

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Why is timekeeping with such unprecedented accuracy required? Does this affect our daily lives? Scientists say such meticulous timekeeping is important even for our day-to-day activities, though we do not realise it.

Redefining the second

Let’s take the case of redefining the second — as planned by the General Conference on Weights and Measures under the Paris-based Bureau International des Poids et Mesures that updates all international measurement units periodically. The last such exercise happened in 2018 when it redefined the unit of “mass”.

The metrology body’s next target is to re-standardise the “second” in 2026 or in 2030 depending on the availability of complex optical clock technologies.

Before 1960, a second was defined as the fraction 1/86,400 of the mean solar day with the exact definition of "mean solar day" left to astronomers. But subsequent measurements revealed irregularities in the rotation of the earth following which the unit has been defined on the basis of the accurate electronic oscillation properties of Caesium-133 atoms.

With optical clocks, the measurement can be even more precise as they are three orders of magnitude better than the best Caesium clocks.

Such precise measurement is vital because an inaccurate measurement of the basic unit of time can lead to wrong spatial positioning on any navigational aids, including the ubiquitous Google map. The implications range from international travel to space missions and missile firing.

“Ultra high-accuracy time and frequency measurements have several applications that profoundly impact current-day society, from day-to-day life to strategic sectors and exploring fundamental science,” said Subhadeep De, a scientist at Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, and leader of one of the Indian teams developing such a clock.

The most important of these applications pertain to satellite-based navigation, communication, surveillance, space missions, e-commerce, digital archiving, meteorology, automatisation in transport, stock markets, smart power grids, industry 4.0, the Internet of Things and emerging quantum technologies, he adds.

The three other groups pursuing the technology are from Council of Scientific and Industrial Research's National Physical Laboratory (NPL) in Delhi, Indian Institute of Technology, Tirupati and Indian Institute of Science, Education and Research, Pune.

The IUCAA and Indian Institutes of Science Education and Research (IISER) teams have also proposed to set up a four-km long unique fibre optic link that will be vital for users like ISRO and the armed forces.

The four institutes are trying different approaches to realise such clocks. Also, at least three clocks of similar accuracy are needed to establish the quality of their performance. This is what scientists call the “three-corner hat measurement” technique: The accuracy and stability of one optical clock can be measured and guaranteed by comparing it with two other optical clocks.

There are two most common types of optical atomic clocks — the type using a single atomic ion and the type using an ensemble of neutral atoms. With improvements in laser cooling, trapping and readout techniques, it has become possible to manipulate atoms and/or ions at a single quantum level in a well-controlled environment. But there are multiple technical challenges that the scientists need to overcome. Besides, these are expensive technologies, requiring Rs 15 to 20 crore for developing one such clock.

Immediate applications

Within India, the most immediate applications for optical clocks will be updating Indian Standard Time — the national primary timescale maintained by the NPL and the Indian Regional Navigation Satellite System Network Timing centres in Bengaluru and Lucknow. Other uses will include synchronising land and space-based distributed defence systems for secure and glitch-free operations of Doppler radar systems; wireless communication; and navigation in hostile terrain or underwater where GPS signals may not be available.

It will also aid meticulous data collection by radio telescopes like the Giant Metrewave Radio Telescope in Pune and Ooty radio telescopes, as well as time synchronisation among worldwide gravitational wave detectors, including the upcoming Laser Interferometer Gravitational-Wave Observatory-India (LIGO-India) observatory.

“With the advent of sophistication in technologies based on quantum phenomena, the requisite levels of time, frequency and phase synchronisation, and time stamping among the distributed devices are becoming more and more stringent. These can be met only by optical atomic clocks,” said De, noting that such clocks have been identified as an indispensable critical technology in quantum missions worldwide, including in India.