Time is an inalienable part of our reality. Scientists don’t understand it fully at the universe’s largest and smallest scales, but fortunately for humans, a panoply of natural philosophers and inventors have allowed us to keep step with its inexorable march — with clocks.

What is a clock?

Clocks are devices that measure the passage of time and display it. Their modern versions have the following parts: power source, resonator, and counter.

A clock measures the amount of time that has passed by tracking something that happens in repeating fashion, at a fixed frequency. In many modern clocks, for example, this is a quartz crystal. More rudimentary devices often depended on natural events instead.

The sundials in use in ancient times allowed people to ‘tell’ time by casting shadows of changing lengths against sunlight. In water clocks, water would slowly fill a vessel, with its levels at different times indicating how much time had passed. The hourglass served a similar purpose, using sand instead of water.

How did mechanical clocks work?

Until the Middle Ages, engineers around the world improved the water clock with additional water tanks, gear wheels, pulleys, and even attached musical instruments to the point where they were practically developing rudimentary analog computers.

One of the first major revolutions in timekeeping that paved the way for modern clocks was the invention of the verge escapement mechanism in the 13th century, which first opened the door to mechanical clocks. The fundamental element here was a gear that, through a combination of mechanical arrangements, could only move in fixed intervals. The gear was called an escape wheel if it was circular. A second gear, called the balance wheel, enmeshed with the first such that when the escape wheel moved forward one gear tooth at a time, the balance wheel would oscillate back and forth. This oscillation would drive the ‘hands’ of a clock on a clockface as long as some force was applied on the balance wheel to keep it moving.

Using an escapement mechanism, a clock-maker named Giovanni Dondi dell’Orologio built a sophisticated instrument called the ‘Astrarium’ over 16 years in the mid-14th century to track the motion of stars and planets in the sky.

Between the 15th and 18th centuries, clockmakers developed and improved on spring-driven clocks. These devices replaced the suspended weight that applied the force on the balance wheel in the previous designs with a coiled spring. To keep these clocks from becoming inaccurate as the spring unwound, clockmakers also developed mechanisms like the fusee, which ensured the spring always delivered a uniform force. The idea to couple a balance spring with the balance wheel also led to the advent of pocket watches.

After every ‘tick’ motion before the ‘tock’ motion towards the other side, the balance spring would return the balance wheel to its neutral position. As a result, the clocks lost a few minutes a day versus a few hours a day before.

Finally, in the mid-17th century, the Dutch inventor Christiaan Huygens invented the pendulum clock. While the clock itself used the by-then familiar escapement mechanism, Huygens made an important contribution by working out a formula to convert the pendulum’s swings to the amount of time passed.

How did clocks change shipping?

The marine chronometer came the next century. For a ship to accurately know where it was on the face of the earth, it needed to know its latitude, longitude, and altitude. The latitude could be computed based on the Sun’s position in the sky and the altitude could be assumed to be sea level, leaving the longitude — which requires an accurate clock onboard each vessel. Pendulum clocks couldn’t serve this purpose because the ship’s rocking motion rendered them inaccurate.

A carpenter named John Harrison built a working marine chronometer in 1761 and delivered it to the British government for its longitude prize, worth GBP 20,000 at the time (and more than Rs 21 crore today). This device featured mechanisms that ensured the clock’s operation wasn’t affected by the ship’s rocking, the force of gravity, and some temperature changes.

Thus, time flew until modernity dawned. The better clocks of the 19th century were electric clocks, i.e. whose energy source was a battery or an electric motor rather than suspended weights or springs, although the former and latter were attached to improve the efficiency of existing designs. And at long last came the 20th century.

How do quartz clocks work?

Two important types of clocks in operation today are the quartz clock and the atomic clock. The fundamental setup of both these instruments is similar: they have a power source, a resonator, and a counter.

In quartz clocks, the resonator is a quartz crystal. The power source sends electrical signals to a quartz crystal, whose crystal structure oscillates due to the piezoelectric effect. The signal’s energy can be tuned to make the crystal oscillate at its resonant frequency, making it the resonator. The counter counts the number of periodic oscillations and converts them into seconds (depending on the crystal’s period). A digital display shows the counter’s results.

Such quartz clocks are inexpensive to make and easy to operate — and their invention led to watches and wall-clocks becoming very common from the mid-20th century.

What are atomic clocks?

An atomic clock may seem futuristic in comparison. The power source is a laser and the resonator is a group of atoms of the same isotope. The laser imparts just enough energy for the atom to jump from its low energy state to a specific higher energy state. And when the atom jumps back down, it releases radiation with a well-established frequency. For example, the caesium atomic clock uses caesium-133 atoms as the resonator. When these atoms excite and then de-excite, they release radiation of frequency 9,192,631,770 Hz. So when the counter detects 9,192,631,770 full waves of the radiation, it will record that one second has passed.

Atomic clocks are distinguished by their resonator; each such clock is called a time standard. For example, India’s time standard is a caesium atomic clock at the National Physical Laboratory, New Delhi, which maintains the Indian Standard Time.

Many countries are currently developing next-generation optical clocks. This is because the higher the frequency of the radiation emitted in the clock, the more stable the clock will be. That emitted in a caesium atomic clock is in the microwave range (gigahertz), and the resulting clock loses or gains a second only once in 20 million years or so. The radiation in the next-generation clocks is in the optical range (hundreds of terahertz) — thus the clocks’ name. These devices use strontium or ytterbium atoms as resonators and don’t miss a second in more than 10 billion years.

Some physicists have even started work on the next-to-next generation of devices, called nuclear clocks: their resonators are the nuclei of specific atoms rather than the whole atom. Atomic clocks need to make sure the resonator atoms aren’t affected by energy from other sources, like a stray electromagnetic field; an atom’s nucleus, however, is located well within each atom, surrounded by electrons, and thus could be a more stable resonator.

Since April this year, researchers around the world have reported three major developments in building functional nuclear clocks: a laser to excite thorium-229 nuclei to a specific higher energy state, a way to link a thorium-229 nuclear clock with an optical clock, and a precise estimate of the excitation energy. The nucleus’s de-excitation emission has a frequency of 2,020 terahertz, alluding to an ultra-high precision.

Atomic clocks are the backbone of the Global Positioning System (GPS), the network of satellites above the earth that we use every day to navigate cities, respond to emergencies, and organise military operations, among other things.

Despite being one of the most accurate timekeeping methods, however, there is still room for improvement. Scientists today are pushing the boundaries with a new technology called optical atomic clocks.

But for being such sophisticated instruments, both these clocks are also bulky, power-hungry, fragile, and expensive (see image below). As a result, their installation and operations are often restricted to big research facilities.

A study recently published in the journal Nature introduced a kind of portable optical atomic clock that can be used onboard ships. While these devices traded some accuracy for size and robustness, they were still more accurate than other vessel-borne timekeeping options.

According to the researchers, this is the most performant optical clock based at sea and represents a significant advancement in optical timekeeping. 

The working of an atomic clock

Atomic clocks work by keeping time using atoms. One popular design uses atoms of an isotope of caesium, Cs-133. The International Committee for Weights and Measures first used it in 1967 to define the duration of one second. India also uses a Cs-133 atomic clock to define the second for timekeeping within its borders.

Cs-133 is a highly stable atom and is found naturally, which is why it is so commonly used in atomic clocks.

Atomic clocks exploit a fundamental property of all atoms: their ability to jump between different energy levels. Energy levels are like the steps of a ladder. An atom climbs up the ladder by absorbing energy, like electromagnetic radiation.

In a Cs atomic clock, the energy needed for the atom to jump to a higher energy level matches the frequency of microwave radiation. This frequency is related in some fully understood way to the duration of a second.

First, researchers keep the Cs atoms in a cavity, to which microwave radiation of a specific frequency is applied. When the frequency of this radiation matches the transition energy of the Cs atoms, the match-up is called a resonance.

The Cs-133 atoms absorb this radiation and jump to a higher energy level. This transition only happens when the frequency of the applied radiation is equal to 9,192,631,770 Hz.

Put another way, when the Cs-133 atom completes 9,192,631,770 oscillations between the two energy levels, one second will have passed.

The accuracy of atomic clocks comes from a feedback mechanism that detects any changes in the resonance frequency and adjusts the microwave radiation to maintain resonance.

Thus, a caesium atomic clock loses or gains a second every 1.4 million years.

How optical atomic clocks are different

Optical atomic clocks are even more accurate. While they have the same working principle, the resonance frequency here is in the optical range. Radiation in this range includes visible light (to humans) and ultraviolet and infrared radiation.

As part of an optical atomic clock, researchers use lasers to stimulate atomic transitions. The lasers’ light is highly coherent: the emitted light waves all have the same frequency and their wavelengths are related to each other in a way that doesn’t change. The result is light with more precise properties and great stability.

Optical atomic clocks use coherent light to achieve higher accuracy in two main ways.

The first is the higher operating frequency of atomic clocks. Say we have two clocks, A and B. A has a higher operating frequency than that of B — which means A will complete more oscillations than B in the same time.

As a result, A will be able measure smaller increments of time more accurately because it has more cycles to count within that time frame.

The second reason is that optical atomic clocks have much narrower linewidths. The linewidth is the range of frequencies over which the transition occurs. The narrower the linewidth, the easier it is to tune the frequency of the optical light that produces the resonance. This leads to higher accuracy because it enables more precise changes.

The most commonly used atom in optical atomic clocks is strontium (Sr): it has narrow linewidths and stable optical transitions.

Researchers at the Indian Institute of Science Education and Research, Pune, are working on a strontium optical atomic clock. Their peers at the Inter-University Centre for Astronomy and Astrophysics in the same city are developing a similar clock with ytterbium ions. These devices, once ready, will bring precision timekeeping in India to the optical regime.

Building a portable optical atomic clock

The researchers in the Nature study developed an optical atomic clock that uses molecular iodine as the frequency standard.

Traditional optical atomic clocks are large and not easy to transport. Scientists have previously attempted to make them portable. Those in the new study wished to fit them within a standardised rack, of the sort used in data centres, laboratories, and telecommunications facilities.

To do this, the team miniaturised the clock’s spectrometer, laser system, and frequency comb.

The spectrometer, used to measure the frequencies of transitions, was designed to have a volume of 2.5 litres. The laser system was built using optical fibres — slender, flexible, transparent cables made of glass or plastic and which can transmit light over long distances. The system thus had a volume of only 1 litre, and operated with light of wavelength 1,064 nm.

A frequency comb is a device that generates a series of equally spaced optical frequencies. This provides a stable and accurate reference for tracking the atomic transitions and generating precise optical frequencies. The frequency comb occupied a volume of 0.5 litres.

The researchers also equipped the clock with a software control system that could autonomously initialise the clock from an ‘off’ state to a fully operational state. It monitors temperature, identifies specific transitions, activates some components, and ensures the system stays stable by continuously checking for problems.

As a result, the final clock had a total volume of 35 litres — about the size of a large backpack. It weighed around 26 kg and consumed 85 W of power, which is just above the consumption of an incandescent light bulb.

Optical atomic clocks at sea

The researchers conducted initial tests at the U.S. National Institute of Standards and Technology (NIST) in April 2022. They operated two prototypes, called PICKLES and EPIC, autonomously for 34 days.

The optical atomic clocks’ accuracy fluctuated less over short periods, outperforming NIST’s hydrogen maser ST05, one of the world’s most accurate and stable atomic clocks, which is based on hydrogen atoms.

The optical atomic clocks also had 10x lower long-term drift compared to rubidium atomic clocks. This means that over long periods, the rate at which the clock’s frequency changes is much lower compared to changes in rubidium atomic clocks. It is a sign of the compact clock’s high stability.

The researchers also deployed the two clocks plus another, called VIPER, on a boat at Pearl Harbor in Hawaii to test them at sea. VIPER was built with a smaller spectrometer and a more simplified laser design, according to the paper.

Despite the ship’s motion, a temperature fluctuation of 2-3 degrees C, and 4-5% changes in humidity, the clocks were nearly as stable as they were in laboratory conditions for up to 1,000 s at a time. When operated for more than a lakh seconds at a time, the clock was still highly stable but also more susceptible to being affected by temperature fluctuations.

Accuracy trade-offs and applications

Atomic clocks are prized for their accuracy, losing or gaining just one second over 300 million years. Optical atomic clocks only lose or gain a second over 300 billion years.

The new iodine clock isn’t as accurate as an optical atomic clock in the laboratory, trading it off for mobility and robustness. But it is still accurate enough to lose or gain a second only every 9.1 million years.

The development of such setups is a necessary first-step for their use for navigation, maritime communication, and scientific research. For example, they can now help monitor underwater seismic and volcanic activity with great precision.

Onboard spacecraft, they can help scientists conduct experiments that test the theories of relativity and potentially reduce the cost of satellite-based navigation.

Tejasri Gururaj is a freelance science writer and journalist with a master’s degree in physics.