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Timekeeping with Quantum Technology

See how the science of timekeeping is evolving.

Breakthroughs in technology are requiring ever more precise timekeeping and time synchronisation techniques. Scientists continue to look toward quantum technology for increasingly innovative technical solutions.

The quest for accurate timekeeping has pre-occupied humanity for the majority of its existence. At least initially, the ability to accurately tell the time was often a matter of life and death – in the classical period, the right placement of the stars in the sky told Greek farmers when to sow a crop or when to hew wood, and the fate of empires rested on what a politician or lawyer could orate within the strict limits of the water clock in a political forum.

As humanity moved into the enlightenment and industrial revolution, the development of an accurate marine chronometer allows sailors to navigate the seas far more safely, savings thousands of lives and even more thousands of pounds in trade. As chronometers became ever more precise, more people became synchronised across time zones; bureaucracy, commercialisation, and globalisation consolidated the world at large allowing modern society to develop and thrive.

And now, as we race through the 21st century, breakthroughs in technology demand ever more precise timekeeping and time synchronisation.

Global Positioning Satellite (GPS) navigation, for example, requires the precise co-ordination of light beams between satellites and the ground to pinpoint a location; experiments to probe the fundamental properties of gravitational waves require extremely precise time synchronisation between arms of the instrument, and quantum communication protocols require instruments to precisely differentiate photon detections occurring at picosecond intervals.

These problems find innovative solutions in systems that take advantage of – and, in fact, demand – the exploitation of quantum physics.

As just one well-known example, the atomic clock has been successfully used for years. This type of early quantum clock works by taking advantage of the quantised energy states of an atom. Particular atoms will be excited into a higher energy state by a specific resonant frequency. Caesium 133 atoms, for example, have a resonant frequency of 9,192,631,770 Hz (i.e. this many oscillations per second).

If a collection of Caesium 133 atoms is subjected to microwave radiation and then passed through an apparatus to measure their energy state, the number of atoms observed in the excited state will be at maximum when the radiation is exactly at the resonant frequency of the atom. By precisely tuning the microwave radiation frequency to the point at which maximum excitations are observed, we can determine the resonant frequency with corresponding precision. This resonant frequency can then be used to determine the length of a second (i.e. “as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium 133 atom”). Since every caesium atom will always have the same resonant frequency, by performing this measurement we can obtain a highly accurate measurement of the second regardless of where or how the clock is made.

However, caesium 133 atoms are still subject to instabilities, such as from temperature, microwave interference and atomic collisions.

More recent developments have realised the use of optical clocks, in which light is used instead of microwaves. Light, having a frequency 100,000 times more than caesium, means that the clock can be correspondingly more precise. The key to producing an optical clock is to find a highly stable atom (such as Strontium or Ytterbium) that demonstrates an energy transition at the desired frequency (an unstable atom would broaden the resonant frequency and lower the precision of the clock). An atom can be suitably stabilising by ionising it, trapping it in an electromagnetic trap, and cooling it to mK temperatures.

The disadvantage of a single atom optical clock is that the atom must be charged to be trapped in an electric field, meaning that multiple atoms cannot be captured and interrogated simultaneously without perturbing the clock frequency. Current research and development is being directed into a more stable version of the optical clock, the optical lattice clock, which utilizes standing waves of lasers to trap neutral atoms. Interactions between neutral atoms are fairly short ranged, so many atoms can be trapped and interrogated simultaneously and significantly improve the accuracy of the clock. Indeed, a recent optical lattice clock has been developed that loses only one second every 300 billion years.

Quantum innovation in timekeeping does not stop at the development of more precise atomic oscillators – other quantum properties such as the “spooky” concept of entanglement has been proposed to provide a common phase reference between remote quantum clocks and allow for very precise synchronisation.

We are now reliant on technology that requires us to understand, measure and keep time with precision that was completely unimaginable to our scientific forebears. Yet as our technology develops to solve one set of technical problems, entirely new ones emerge that demand even more solutions. More and more time-keeping precision is going to be required as we grabble with more questions of fundamental physics, and become even more reliant on advanced technology. Luckily for us, however, the universe (and quantum physics) is quite ready to provide the answer if we are willing to look hard enough.

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