Keep up with time as accurately as possible with atomic clocks

Atomic clocks are known to be the most accurate in terms of timekeeping, but how do they do it?
Raymond Aguilar
Atomic clock
Atomic clock

Wikimedia Commons  

As the old saying goes, “Time is golden.” A bit cliche, perhaps, but true. Time is such an important aspect of our everyday lives that every hour, if not every second, matters. Whether it is from clocking in to begin work to navigating in space, the measurement of time is vital.

Just like anything that needs to be measured, one of the most important aspects of time is its accurate measurement. Think of questions such as “how accurate is the time being displayed right now?” or “how is a second in time defined? “ Such questions would bring us to the basics of timekeeping, and what better way to understand it if not by learning what an atomic clock is?

What is an Atomic Clock?

Atomic clocks are known to be the most accurate clocks in terms of timekeeping. These clocks work to measure time by using the frequency of atom vibrations. In 1967, the International System of Units (SI) redefined the second as the time it takes a cesium-133 atom to release 9,192,631,770 cycles of microwave radiation when vibrating between two energy states.

Cesium atomic clocks can produce a frequency so precise that its time error per day is about 0.03 nanoseconds, which means that the clock would lose one second in about 100 million years.

Keep up with time as accurately as possible with atomic clocks
NIST researcher David Leibrandt and the quantum logic clock

How do Atomic Clocks work?

The mechanics of how an atomic clock works can be best learned by knowing how mechanical clocks work in the first place. A clock works by counting periodic events with a known frequency reference. For example, in a grandfather clock, the pendulum becomes the frequency reference, with each half-swing taking up a second in time.

A wristwatch works in a similar manner – with the difference of using quartz crystal as the frequency reference. What happens is that a piece of the crystal is placed in an electronic circuit where 32,768 oscillations are equal to one second. Compared to the grandfather clock, the higher number of oscillations in a quartz crystal increases its accuracy in telling time.

In an atomic clock, atoms are used similarly by measuring the frequency of oscillations of their electrons. A quartz crystal oscillator is still in use in an atomic clock, just like in a wristwatch, but the movement of electrons in atoms, usually cesium-133 atoms, is used to regulate the quartz oscillator.

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The quartz oscillator in an atomic clock is set according to the SI definition of a second, with a frequency of 9,192,631,770 Hz. That is 9,192,631,770 cycles of the radiation that gets an atom of the element called cesium to vibrate between two energy states.

The higher and lower energy states of cesium atoms determine if the quartz oscillator is in its correct frequency. Let’s say that we have a flow of cesium atoms in their lower energy state, and this stream of atoms passes through a magnetic detector. This detects if the stream mostly has cesium atoms in either a higher or lower energy state, which will indicate if there are any changes in the quartz oscillator’s frequency.

Microwave radiation from a resonator energizes cesium atoms, shifting them from a lower energy state to a higher energy state. However, it is important to note that the oscillator would only be able to promote cesium atoms into a higher energy state if the oscillator itself is set to the correct frequency. The percentage of atoms that change their state while passing through the resonator depends on the frequency of the microwave radiation. The more it is synced with the inherent oscillation frequency of the atoms, the more atoms change their state.

The goal is to perfectly tune the microwave frequency to the oscillation of the atoms and then measure it – after exactly 9,192,631,770 oscillations, a second has passed.

Going back to the earlier example, the stream of lower energy state atoms now passes through a magnetic detector. This would indicate that the oscillator is not in the correct frequency as it can no longer generate microwaves that can move atoms into a higher energy state. A feedback mechanism in the clock would now adjust the quartz oscillator back to its correct frequency, generating microwaves that can promote cesium atoms back into a higher energy state. These mechanisms in an atomic clock maintain the correct frequency of 9,192,631,770 Hz in the quartz oscillator for more accurate timekeeping.

Keep up with time as accurately as possible with atomic clocks
The 6th generation of NBS Atomic Frequency Standards.

Another type of atomic clock, which is used by the US National Institute of Standards and Technology (NIST), uses hydrogen atoms. The atoms are excited using radiowaves and then sent into a vacuum chamber. As they decay, they emit a specific frequency of light. Instruments then count the cycles of oscillation in the light. At the NIST, the “ticks” from 21 of these hydrogen clocks are averaged together to count the time to within one quadrillionth of a second.

How atomic clocks are used

Aside from being the most accurate timekeepers, atomic clocks have other uses, even outside Earth. They are being used in several technological applications in different fields like physics, astronomy, and the military, as well as in satellites and navigation systems like GPS.

DARPA's Robust Optical Clock Network (ROCkN) program would bring atomic clocks that are portable and small enough to fit inside military vehicles and satellites. Military operations usually rely on GPS technology, but the integration of a portable atomic clock would upgrade things to another level in terms of increased precision and accuracy. In deep space explorations, atomic clocks are also important in providing information to determine the location of a spacecraft.

Developments in timekeeping

Yet another type of atomic clock being used is the cesium fountain atomic clock. Just like most atomic clocks, this also uses cesium atoms, but it is called a fountain clock because the cesium atoms are ‘tossed’ in the air and fall back.

What happens here is that cesium gas is inserted into a vacuum chamber, where an infrared laser beam pushes the atoms into a ball. At the same time, the atoms are cooled to near absolute zero, which slows them down. Vertical laser beams are then used to “toss” the atom ball upward through a microwave-filled cavity, then the lasers are turned off, and the ball drifts back down from the force of gravity. During their up-and-down trip through the microwaves, the atomic state of some of the cesium atoms is partially altered.  

These atoms will then be hit with another laser, and those that have been altered by the microwaves will emit light or fluoresce – this fluorescence is measured by a detector. The toss and return is repeated using microwave signals tuned to different frequencies until a frequency is found that alters the state of most of the cesium atoms. This is the resonance frequency that is equivalent to the SI definition of a second (9,192,631,779 Hz).

When one thinks of atomic clocks, they are usually associated with large structures that are situated inside laboratories. Although this is true, some advancements today aim to develop different kinds of atomic clocks, including those that come in much smaller sizes, such as NASA’s Deep Space Atomic Clock, which is the size of a toaster oven. This atomic clock is essential in deep space exploration because of how compact it is compared to usual atomic clocks.

Keep up with time as accurately as possible with atomic clocks
NASA's deep space atomic clock

Space exploration requires two-way communication in order to calculate the position of a spacecraft. This requires sending a signal from Earth to the spacecraft and vice versa, where the time it takes to deliver the signal will be used to determine the distance with the help of atomic clocks. The entire process can be lengthy, and the development of NASA’s Deep Space Atomic Clock aims to improve space navigation. This is because this atomic clock would allow a spacecraft to calculate its own location instead of waiting for the calculated position sent from Earth. 

From here, it can be said that the current developments in the field would allow future atomic clocks to be available in smaller, portable forms while maintaining their accuracy, allowing them to be utilized in different fields. A newly-developed clock, called an optical lattice clock, operates on optical frequencies and is thought to be so precise that it is off by just a second every 15 billion years. This clock takes timekeeping to a whole new level, surpassing the accuracy given by atomic clocks. An optical lattice clock works with strontium atoms that are trapped in an optical lattice. Lasers are then used to oscillate several atoms simultaneously, allowing time to be measured faster and more precisely.

How many atomic clocks are there in the world?

There are about 400 atomic clocks around the world, and these clocks maintain time as we know it – making timekeeping as accurate as possible. Since the creation of atomic clocks using cesium-133 atoms, research in the field has allowed them to evolve, with even some atomic clocks being developed to become more precise using different elements and methods.

Atomic clocks work to maintain the accuracy of time. By keeping a constant frequency of oscillations, we now have time that is only off for a second, as little as every 100 million years. Although the measure of time is often overlooked when it comes to daily activities, time has been vital in terms of the developments we have today, as well as in understanding different theories and concepts in science.

Now, whenever you hear the phrase "Time is golden," don’t you think it might be a little more accurate to change it into something like "Time is quartz… or cesium," perhaps?