Want a perfectly soft boiled egg? Time it with a ytterbium atomic clock (Image: Burrus/NIST)

Time can now be divided into slivers hundreds of trillions of times smaller than a second, thanks to a pair of atomic clocks made from ytterbium that have just set a new record for precision.

This could allow us to detect how an object just 1 centimetre above another might age differently, as prescribed by Einstein’s theory of general relativity. It could also set a new standard definition for the second.

“We’ve reached a new level, an order of magnitude improvement over what had been done before,” says Andrew Ludlow of the US National Institute of Standards and Technology in Boulder, Colorado, who led the work.


The “ticks” of atomic clocks are the hyper-regular switching of a group of atoms between two energy levels. The most accurate definition of a second is currently the amount of time it takes for a group of caesium atoms to swing between two states 9,192,631,770 times.

“If you were to run this clock for around 100 million years, it would only gain or lose about a second,” Ludlow says. These clocks are accurate because we’ve identified their sources of error and eliminated most of them, so physicists can be confident that its ticking is true.

Speed limit

But the trouble with caesium is that it can’t switch energy states any faster, limiting the clock’s precision – how finely we can divide time.

In the past few years, physicists have been constructing clocks that use elements like strontium, aluminium and ytterbium, whose transitions are thousands or millions of times more frequent.

“The caesium clocks, compared to most other technology, are wonderful,” Ludlow says. “But compared to these next-generation clocks they are significantly worse in terms of the stability, or the time precision that they can achieve.”

As well as the speed of its tick, a clock’s precision depends on its regularity. If the pendulum in a grandfather clock takes one second to complete one swing, two seconds to complete the next, and a second and a half to complete a third swing, you wouldn’t trust it to time a race. So a clock’s ticking rate must also be consistent. “The same is true for these atomic clocks,” Ludlow says. “You need to make sure that each tick is the same as the one before it.”

Magic frequency

Now, Ludlow and his colleagues have created ytterbium clocks that are stable to one part in a quintillion (1018): it would take a quintillion ticks to find one that is different from its neighbours.

To create each clock, the team cooled 10,000 ytterbium atoms to 10 millikelvin, or 10 thousandths of a degree above absolute zero, and used a series of lasers to trap them in a sort of egg carton of light. Another laser, called the “clock” laser, provoked a transition between two of the atoms’ energy levels.

The magic frequency for ytterbium is about 518 trillion oscillations per second, about 100,000 per second faster than caesium.

The team used an extremely steady laser to reduce jitter in the atoms, and thousands of atoms to average out any disturbances that could have knocked individual atoms off their cycles. To put a figure on the precision, they compared two nearly identical ytterbium clocks against each other.

“It’s an outstanding paper. This is really a breakthrough,” says Christophe Salomon of the École Normale Supérieure in Paris, who was not involved in the new work.

Although the clock is more precise than the caesium gold standard, it still not as accurate.

You might think of these properties of equivalent, but there is a distinction. Precision describes how finely you can divide something, and accuracy is the extent to which you can be sure the measurement is correct or erase systematic errors. And the researchers are more certain that the caesium clock does not contain a systematic error than they are for ytterbium.

Einstein test

If they beat down the clock’s uncertainties, it could eventually become more accurate than the caesium clock too, potentially unseating the world standard for timekeeping.

The milestone opens new frontiers in ultra-precise measurements of gravity and fundamental constants.

It could also help find holes in general relativity. The theory predicts that time runs slower in a gravitational field, meaning clocks on the ground are slower than clocks in space, or even clocks on a stepladder.

Using the ytterbium clock’s precision, you could sense these time differences at the level of a single centimetre. That would allow physicists to test general relativity’s predictions to 10 parts per billion, well beyond what has been done so far.

Relativity, although hugely successful, doesn’t sit well with quantum mechanics, so physicists expect it to break down at some point, revealing a new, more fundamental theory.

“We know that general relativity is not the ultimate theory,” Salomon says. “People are searching for violations of general relativity that would indicate new forces or new particles or new physics, and that would be really exciting. These [atomic clocks] are exquisite tools for doing that work.”

Journal reference: Science, DOI: 10.1126/science.1240420