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Home , Atomic clock, Louis Essen, Quartz clock

physicsworld.com Measurement: iStock/Greyfebruary

A brief of timekeeping From sticks in the ground to atomic , humans have been keeping track of time with increasing accuracy for millennia. Helen Margolis looks at how we reached our current definition of the , and where technology is going next

On 1 November 2018, when this article is published, I placing a stick upright in the ground and keeping Helen Margolis is a will have been working at the UK’s National Physical track of its moving shadow as the progressed. fellow in optical Laboratory (NPL) in Teddington for exactly 20 This method evolved into the , or shadow standards and six days. The reason I know this is easy – I joined clock, with markers along the shadow’s path dividing and at the on 26 October 1998 and, with the help of clocks and the day into segments. National Physical , I can measure the time that’s passed. But However, are useless unless the Sun is Laboratory in what did people do before clocks came about? How shining. That’s why mechanical devices – such as Teddington, UK, e-mail helen. did they measure time? water clocks, candle clocks and – were [email protected] Over the millennia a myriad of devices has been developed. Then, in the 17th , invented for timekeeping, but what they all have in clocks were developed, which were far more accu- common is that they depend on natural phenomena rate than any preceding timekeeping devices. Their with regular periods of . Timekeeping period of oscillation (in the lowest-order approxi- is simply a matter of counting these to mation) was determined by the acceleration due to mark the passage of time. gravity and the length of the pendulum. Because this For much of history, the chosen periodic phenom- period is far shorter than the daily rotation of the enon was the apparent motion of the Sun and stars Earth, time could be subdivided into much smaller across the sky, caused by the Earth spinning about intervals, making it possible to measure , or its own axis. One of the earliest known timekeeping even fractions of a second. methods – dating back thousands of years – involved Nevertheless, the Earth’s rotation was still the

Physics World November 2018 27 Measurement: Time physicsworld.com

Standardizing time (see box on p30). Their device was not truly a clock as it did not run continuously, and was simply used to is not the same everywhere. In the UK, for example, Birmingham calibrate the frequency of an external clock at is eight behind London, and Liverpool is 12 minutes behind. While intervals of a few days. Nevertheless, by studying how communication and travel between major centres of population were slow, the frequency depended on environmen- this mattered little. But the situation changed dramatically with the construction tal conditions, Essen and Parry had shown convinc- of railways in the 19th century. Having different local times at each station ingly that transitions between discrete energy levels caused confusion and increasingly, as the network expanded, accidents and near in well-isolated caesium could provide a much misses. A single standardized time was needed. more stable time-interval reference than any stand- The Great Western Railway led the way in 1840 and “railway time” was ard based on the motion of astronomical bodies. As gradually taken up by other railway companies over the subsequent few years. Essen later wrote: “We invited the director [of NPL] Timetables were standardized to Mean Time (GMT), and by 1855 time to come and witness the death of the astronomical signals were being transmitted telegraphically from Greenwich across the British second and the birth of atomic time.” railway network. However, it was not until 1880 that the role of GMT as a unified But showing that the new standard was stable was standard time for the whole country was established in legislation. Four years insufficient to redefine the second. A new defini- later, at the International Meridian Conference held in Washington DC in the US, tion had to be consistent with the old one within the GMT was adopted as the reference standard for time zones around the globe and technical limit of measurement uncertainty. Essen the second was formally defined as a fraction (1/86 400) of the mean solar day. and Parry therefore proceeded to measure the fre- quency of their caesium standard relative to the astronomical timescale disseminated by the Royal “master clock” against which other clocks were cali- Greenwich Observatory. brated and adjusted on a regular basis. In the meantime, astronomers had switched to using , based on the orbital period of From crystal to atomic the Earth around the Sun. Their rationale was that As technology progressed, the need for higher-reso- it is more stable than the Earth’s rotation, but unfor- lution timing increased. Pendulum clocks were grad- tunately for most practical measurement purposes it ually overtaken by quartz clocks, the first of which is impractically long. Nevertheless, the International was built in 1927 by Warren Marrison and Joseph Committee for Weights and Measures followed their Horton at the then Bell Telephone Laboratories in lead, and in 1956 selected the ephemeris second to the US. In these devices, an electric current causes a be the base in the International System quartz crystal to resonate at a specific frequency that of Units. As Essen put it: “Even scientific bodies can is far higher than a pendulum’s oscillations. make ridiculous decisions.” The frequency of such clocks is less sensitive to But ridiculous or not, he needed to relate the environmental perturbations than older timekeep- caesium frequency to the ephemeris second, a task ing devices, making them more accurate. Even so, he accomplished in collaboration with William quartz clocks rely on a mechanical vibration whose Markowitz from the United States Naval Observa- frequency depends on the size, shape and tempera- tory. Finally, in 1967 the General Conference on ture of the crystal. No two crystals are exactly alike, Weights and Measures decided that the time had so they have to be calibrated against another refer- come to redefine the second as “the of ence – this was the Earth’s period of rotation, with 9 192 631 770 periods of the radiation corresponding the second being defined as a 1/86 400th of the mean to the transition between the two hyperfine levels of solar day (see box above). the ground state of the caesium-133 ”. There are problems with this definition of the sec- ond, however. As our ability to measure this unit The next generation of time improved, it became clear that the Earth’s More compact and less costly – albeit less accu- period of rotation is not constant. The period is not rate – versions of caesium atomic clocks have also only gradually slowing down due to tidal friction, but been developed, and applications have flourished. also varies with the and, even worse, fluctu- We may not always realize it, but precision timing ates in unpredictable ways. underpins many features of our daily lives. Mobile In 1955 NPL set in motion a revolution in time- phones, financial transactions, the , electric keeping when and Jack Parry produced power and global navigation satellite systems all rely the first practical caesium atomic frequency standard on time and frequency standards. But although the caesium transition has proved an enduring basis for the definition of the second, cae- Although the caesium transition has sium atomic clocks may now be reaching the limit of their accuracy and improvements may open up proved an enduring basis for the new applications. In response, a new generation of atomic clocks is emerging based on optical, rather definition of the second, caesium than , transitions. These new clocks get their improved precision from their much higher atomic clocks may now be reaching operating . All other things being equal, the stability of an is proportional to its operating frequency and inversely proportional to the limit of their accuracy the width of the electronic transition. In practice,

28 World November 2018 physicsworld.com Measurement: Time NPL NPL

though, the stability also depends on the signal-to- recently demonstrated a stability of one part in 1018 Then and now noise ratio of the atomic absorption feature. for averaging times of a few thousand seconds. How- Jack Parry and Louis In an optical atomic clock, an ultra-stable is ever, trapped- optical clocks have also demon- Essen developed locked to a spectrally narrow electronic transition strated stabilities well below those of caesium atomic their caesium in the optical region of the spectrum – the so-called clocks, and both types have now reached estimated frequency standard “clock transition”. The optical clocks being studied systematic uncertainties at the low parts in 1018 level. in 1955 (left) but scientists today are today fall into two categories: some are based on sin- This far surpasses the accuracy of caesium primary focusing on optical gle laser-cooled trapped and others are based standards and raises an obvious question: is it time clocks (right). on ensembles of laser-cooled atoms trapped in an to redefine the second once again? . The former, a single laser-cooled ion in a ­ The of time frequency electromagnetic trap, comes close to the The frequency of the selected optical standard would, spectroscopic ideal of an absorbing particle at rest of course, need to be accurately determined in terms in a perturbation-free environment. When cooled, of the caesium frequency, to avoid any discontinuity it can be confined to a region of with dimen- in the definition. But this can easily be accomplished sions less than the of the clock laser , using a optical – a laser which means Doppler broadening of the absorption source whose spectrum is a regularly spaced comb feature is eliminated. of frequencies – to bridge the gap between the opti- By controlling its residual motion to ensure it is cal and the microwave frequencies. One obstacle to tightly confined to the trap centre, other systematic a redefinition is that it is unclear which optical clock frequency shifts can also be greatly suppressed. This will ultimately be best. Each system being studied has type of clock therefore has the potential for very high advantages and disadvantages – some offer higher accuracy. The drawback is that a single ion gives achievable stability, while others are highly immune an absorption signal with low signal-to-noise ratio, to environmental perturbations. which limits the clock stability that can be achieved. Another challenge is to verify experimentally their Neutral atoms, on the other hand, can be trapped estimated systematic uncertainties through direct and cooled in large numbers, resulting in a signal comparisons between optical clocks developed inde- with far better signal-to-noise ratio. Stability, for pendently in different laboratories. Here researchers example, improves with the square root of the num- in Europe have an advantage as it is already possi- ber of atoms, all else being equal. Researchers can ble to compare optical clocks in the UK, France and now confine thousands of laser-cooled atoms in an Germany with the necessary level of accuracy using optical lattice trap – most commonly a 1D array of optical-fibre links. Unfortunately, these techniques potential wells formed by intersecting laser beams. cannot currently be used on intercontinental scales One might expect that the light beams used to and alternative ways to link to optical clocks in the trap the atoms would alter the frequency of the clock US and Japan must be found. transition. However, this can be avoided by tun- Remote clock-comparison experiments must also ing the laser used to create the lattice to a “magic” account for the of the clock fre- wavelength at which the upper and lower levels of the quencies. For optical clocks with uncertainties of one clock transition shift by precisely the same amount – part in 1018, this means the gravity potential at the clock a solution first proposed in 2001 by Hidetoshi Katori, sites must be known with an accuracy corresponding from the University of Tokyo in Japan. to about 1 cm in height, a significant improvement The current record for optical clock stability is held on the current state of the art. Tidal variations of the by Andrew Ludlow’s group from the US’s National gravity potential must also be considered. Institute for Standards and Technology in Boul- Although all these challenges are likely to be over- der, Colorado. Their optical lattice clock come given time, a redefinition of the second will

Physics World November 2018 29 physicsworld.com

How an atomic clock works

microwave 9.192 631 770 GHz source

frequency Cs control all caesium atoms are identical

In a caesium atomic clock, the frequency of a microwave source is carefully adjusted until it hits the resonance frequency corresponding to the energy difference between the two ground-state hyperfine levels of the caesium atoms: 9 192 631 770 Hz. The atoms absorb the microwave radiation, and a signal generated from the absorption signal is used to keep the microwave source tuned to this highly specific frequency. The time display is generated by counting electronically the oscillations of the microwave source. Louis Essen’s original clock at the UK’s National Physical Laboratory used a thermal beam of caesium atoms and was accurate to about one part in 1010. Nowadays, caesium primary standards use an High voltage accelerator assembly arrangement known as an “atomic fountain”, in which laser-cooled for ion-source atoms are launched upwards through a before falling ø 560 x L 450 mm back under gravity. Using cold atoms means the interaction time can be far longer than in a thermal beam clock, giving much higher spectral resolution. With careful evaluation of systematic frequency shifts arising from environmental perturbations, today’s best caesium fountains have reached accuracies of one part in 1016, though measurements must be averaged over several days to reach this level. They contribute as primary standards to International Atomic Time (TAI).

Problems wanted! require international consensus and is still some way off. Until then, the global time and frequency metrol- ogy community has agreed that optical atomic clocks can in principle contribute to international time- scales as secondary representations of the second. Indeed, the unprecedented precision of optical atomic clocks is already benefiting fundamental physics. For example, improved limits have been FRIALIT®-DEGUSSIT® set on current-day time variation of the fine struc- HigH Performance ture constant (α ≈ 1/137) and the proton-to- ratio by comparing the frequencies of different ceramics clocks over a period of several years. Optical clocks could also open up completely new applications. By comparing the frequency of a trans- Expert for accelerator parts made portable optical clock with a fixed reference clock, of high performance ceramics we will be able to measure gravity potential differ- ences between well separated locations with high sensitivity, as well as high temporal and spatial reso- www.friatec.com lution. Such measurements will lead to more consist- ent definitions of heights above sea level – currently different countries measure relative to different tide gauges, and sea level is not the same everywhere on Earth. They could also allow us to monitor changing sea levels in real time, tracking seasonal and long- trends in ice-sheet and overall ocean- mass changes – data that provide critical input into models used to study and forecast the effects of climate change. It is ironic perhaps that we will be able to study the Earth – whose rotation originally defined the second – in greater detail with the help of its latest usurper: the optical clock.  n

30 Physics World November 2018