Clocks Galore

Atomic clocks come in many shapes and sizes. While physicists have pushed the limits of measurement with exquisite fountain clocks and optical clocks, others have designed clocks for the military, banks, telecom companies and other users who need highly accurate — but not necessarily the world’s most accurate — time.

Rubidium Clock: A Workhorse With Many Uses

For precisely measuring the second and keeping accurate and stable time, there’s no beating a cesium or optical clock. But for those who need a more versatile, portable and affordable device, rubidium — essentially a lighter version of cesium — is often the element of choice. When it comes to clocks, rubidium has become the jack-of-all-atoms, ticking off seconds for armies, global businesses, scientific researchers and the masses.

Similar to cesium, rubidium has a loner outer electron that’s easily tweaked with microwaves. In a rubidium clock, a gas of rubidium atoms is trapped in a vapor cell — essentially, a small glass box. A laser puts the atoms into a particular energy state. When the atoms are immersed in microwaves near the rubidium resonant frequency, which is around 6.834 billion cycles per second — slightly more than two-thirds of the cesium resonant frequency — the atoms’ outermost electrons can jump, putting the atom into a higher energy state.

As the microwave source sweeps through a range of frequencies that includes the rubidium resonant frequency, some atoms absorb energy and jump to the higher state. A detector senses changes in the intensity of a light beam passing through the rubidium vapor. The beam’s intensity changes the most when the microwaves are at or very close to the resonant frequency.

A feedback loop locks the microwave oscillator to the resonant frequency of the rubidium atoms. The oscillator’s frequency is then used to drive a counter or digital clock display.

Because they’re small and cheap yet far more accurate and stable than electronic clocks, rubidium clocks have become popular in areas such as telecommunications and science research. They’re probably best known, however, for helping guide the blue circle on your phone that tells you where you are. 

Starting in the 1970s, the U.S. military began sending atomic clocks into space, with the goal of building a global positioning and navigation system with atomic accuracy. Early versions of the GPS system included both cesium and rubidium clocks. Today, the satellites carry only rubidium clocks. Other countries have also put rubidium clocks in their own positioning satellites.

These clocks drift by around a few nanoseconds (billionths of a second) per day. That doesn’t sound like much. But GPS satellites send their signals at light speed, so if the system’s time were off by even 100 nanoseconds, a position estimate would be off by 30 meters, or nearly 100 feet. If GPS clocks were allowed to drift indefinitely, their position estimates would quickly degrade to the point of uselessness. Since modern airplanes use GPS to navigate, this would quickly create dangerous situations. And it would be much harder for us to find our way around. 

The solution is to regularly correct, or steer, the satellite clocks using even better atomic clocks on the ground. This two-tiered system allows the rubidium clock to do what it does best — and help billions of people all over the globe know exactly where they are at any time.

And even as scientists use workhorse commercial rubidium clocks to support and advance diverse areas of science and technology, they are also developing new rubidium clock designs that could ultimately make atomic clocks cheaper and more portable.
 

Hydrogen Maser

A hydrogen maser uses atoms of hydrogen, the lightest chemical element, to keep highly precise time. The term “maser” stands for “microwave amplification by stimulated emission of radiation.” While that may sound like a mouthful, it’s similar to the process that makes lasers work, except that it operates in the microwave frequency range rather than visible light. The maser was actually invented before the laser, though lasers are much more familiar today.

The hydrogen maser starts with molecular hydrogen (H₂) gas that’s put into a tube to create a plasma — a state of matter in which protons and electrons separate from each other and no longer form atoms or molecules. Out of this plasma, atoms of hydrogen, each with one proton and one electron, emerge.

Similar to cesium and rubidium atoms, the electron in a hydrogen atom can jump from lower to higher energy levels and fall from higher to lower levels, and the frequency of light that makes the electron jump is known as the hydrogen resonant frequency. The hydrogen atoms are sent toward a chamber that makes up the heart of the maser. But they first pass through magnetic fields that kick out atoms in the lower energy level, leaving mostly atoms in the higher level.

In the chamber, atoms are bathed in microwaves at the hydrogen resonant frequency. Some atoms spontaneously drop from the higher to the lower energy level, emitting microwaves at the resonant frequency. These microwaves in turn stimulate other excited hydrogen atoms to emit more microwaves, and so on. The hydrogen maser is the only kind of atomic clock that uses this process of stimulated emission to amplify the signal.

Some of the microwave radiation produced inside the chamber is put into a feedback loop to lock the microwave signal to the hydrogen resonant frequency. That signal is sent to an electronic counter that can then be used to drive a clock display.

Hydrogen masers are much larger and more expensive than rubidium clocks, so they’re used less in commercial applications. On short time scales, however, the hydrogen maser frequency is very stable. For that reason, they’re used by measurement labs around the world, including NIST, to help keep official time. Some positioning satellites, such as those of the EU’s Galileo system, also carry hydrogen masers.

Chip-Scale Atomic Clock: Putting Atomic Time in Your Backpack

Not every clockmaker aims to break accuracy and stability records. Some want to make atomic clocks smaller and more efficient — and more useful. That ambition led to the chip-scale clock: the worker bee of atomic clocks. Just as big powerful horses are not suited to every task, small, more nimble clocks can sometimes excel where their larger cousins prove too bulky, power-hungry or costly.

In the 1990s and 2000s, NIST researchers, funded by the Department of Defense and working with several other agencies and companies, started to put atomic clocks on chips. Their goal was to create a small, energy-efficient clock that a soldier could carry in a backpack, power with a battery and use to navigate in places where GPS is unavailable — a growing concern as jamming and spoofing technologies proliferate.

The tiny clocks are based on rubidium atoms enclosed in vapor cells about the size of a quarter that are printed on a silicon wafer. A tiny, low-powered modulated laser measures the atoms’ resonant frequency.

NIST produced the first lab prototypes of the chip-scale atomic clock in 2004. It was one-hundredth the size of previous atomic clocks and consumed one-fiftieth of the power. 

From there, industry took over. In 2011, the company Symmetricom (since acquired by Microchip) manufactured the first commercial chip-scale atomic clocks, launching a quarter-billion-dollar industry. More than 100,000 units have since been sold, mainly to oil and gas companies that use the clocks to synchronize seismic sensors they use to explore for deep-sea deposits. The military has also incorporated chip-scale clocks into tamper-proof GPS receivers.

NIST and other research agencies, meanwhile, have continued developing more accurate and stable miniature atomic clocks. In 2019, the NIST team released an optical clock that’s about 100 times more stable than the earlier, microwave-based chip-scale clock. A partnership among NIST and several companies is now working to commercialize this clock.

Nuclear Clock 

In 2003, physicists came out with an ambitious vision for a new kind of clock that would represent the most revolutionary design change since the beam clock was invented more than three-quarters of a century ago. Instead of tapping energy jumps made by electrons, nuclear clocks would use jumps made by particles called protons and neutrons that are packed inside the atomic nucleus.

These particles, like electrons, can jump from one energy level to another — though rather than just one particle gaining energy, nuclear jumps involve the particles together shifting their collective arrangement to a higher-energy state. Unfortunately, the energies of these jumps tend to be far greater than can be delivered by today’s lasers. 

In its mysterious way, however, the universe has created rare exceptions. One involves a variety — or isotope — of the element thorium, a naturally occurring radioactive metal. The nucleus of thorium-229 has an unusually low-energy jump with a resonant frequency corresponding to light in the ultraviolet region of the electromagnetic spectrum, just beyond the optical frequencies we can see. 

A clock built on such a frequency could chop time into smaller pieces than any clock that exists today, including the best optical clocks. Better yet, an atom’s nucleus is far better shielded from environmental disturbances than are its electrons. That shielding could allow for extremely precise measurements of the nuclear resonant frequency. 

Physicists now anticipate that a so-called nuclear clock could almost automatically become the most stable clock ever invented. Such a clock would enable ambitious experiments that probe the nature of dark matter, gravity and other fundamental aspects of the universe.

Recently, physicists cleared a major hurdle on the path toward building a nuclear clock. In 2024, several research groups reported that they had used lasers to excite quantum jumps within the thorium nucleus. Previously, the energy of the jumps had only been measured indirectly. The new results allow physicists to focus their efforts on developing lasers that can measure the thorium nuclear transition frequency with enough precision to advance beyond existing clocks.

To be sure, a nuclear clock still remains years away. The ultraviolet lasers that are needed to measure the thorium nucleus resonant frequency precisely enough to provide stable beats for a clock will need to improve greatly to match the visible-light lasers used in today’s optical clocks.

But physicists, like the atoms they work with, are a restless bunch. Several research groups are striving to build a new generation of clocks to rule them all. So … stay tuned!