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Physicists are on the brink of redefining time — study

Time is a construct — and it’s made of atoms.


Around the world, we are all controlled by the same, invisible force that tells us when to wake up, when to work, and even when to socialize: time.

After this past year, the concept of time may seem less real than ever, but according to a team of physicists in Colorado, that couldn’t be further from the truth. The team used three different elements to measure the length of a second.

To date, atomic clocks (which absorb and emit photons at regular frequencies to keep time) are the most accurate way to measure the passage of time in seconds, but their accuracy has been stagnated for more than a decade.

Using both optical fibres and invisible laser transmission of data, the research team measured the meaning of a second more accurately than ever before. They did it by looking at minute (the measure of size, not time) differences between time kept by the atoms — a crucial step toward redefining time itself.

What is time?

What’s new — Previous attempts to measure these minute differences between how atoms keep time — also referred to the ratio between them — had only ever delivered an accuracy of up to 17 digits.

But now using their new model, which includes the first-ever use of a ‘free-space link’ for this purpose (essentially, laser pulses of data going through the air instead of a cable,) the University of Colorado's BACON team (Boulder Atomic Clock Optical Network) has now measured this ratio reliably out to 18 digits.

The research was published Wednesday in the journal Nature.

Cesium has long reigned supreme as the element of choice in atomic clocks, but that could soon be changing.Nature

Why it matters — One digit might not make or break a grade on your final exam or even your credit score, but for extremely tiny measurements in physics, this is a big deal.

Rachel Godun is a senior research scientist in the Time and Frequency group at the National Physical Laboratory in the U.K. She wrote an unaffiliated essay on this work also published this week in Nature. She says that the kind of precision demonstrated in this study is literally astronomical.

“Such frequency-ratio measurements are no mean feat, and are equivalent to determining the distance from Earth to the Moon to within a few nanometres,” says Godun in the essay.

The research team reports continued refinement of atomic clock measurements using this model has the potential to redefine the second as we know it and can help physicists test fundamental theories of the universe — including relativity and dark matter — by measuring atomic perturbations even more precisely.

How do we define a second?

Here’s the background — The first atomic clock began ticking in 1949. It was powered by an ammonia molecule, but a cesium isotope quickly became the standard only a few years after.

Since then, scientists have relied on these incredibly precise clocks, which are largely immune to earthly headaches like earthquakes, to help keep precise time. This measurement is used to not only define time itself, but to guide satellites in orbit via GPS as well.

Such a clock, called the “Master Clock,” resides at the U.S. Naval Observatory (USNO) in D.C. In addition to its role as a historic scientific institution, the USNO is also the residence for the vice president of the United States — meaning it’s where Vice President Kamala Harris will live once renovations are complete.

In addition to being home to the U.S. Vice President, the U.S. Naval Observatory is also home to one of the nation’s essential atomic clocks.The Washington Post/The Washington Post/Getty Images

Historically, atomic clocks have worked used cesium to measure fractions of time by counting the jumps the atoms make between different energy states when exposed to certain radio-wave frequencies. Since 1967, the official definition of a second has been “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.”

In other words, there are just over 9 billion cesium energy jumps in one second.

While this method has worked for decades, it is still far from perfect. The oscillation frequency of cesium clocks is in the microwave region of the electromagnetic spectrum (the rainbow that stretches from low energy radio waves to high energy gamma rays and describes all possible frequencies of incoming light.) Newer designs for atomic clocks instead focus on elements whose frequencies would in the optical spectrum (the part we can see) instead. These frequencies would be 100,000 times faster than the microwave range ones emitted from cesium clocks and in turn 100 times more accurate.

But before scientists can think about replacing the cesium in our atomic clocks, they have to prove that other elements would work even better. That’s what the team in Colorado set out to do.

What they did — To better measure time, the researchers used three elemental clocks:

  • an aluminium-ion atomic clock
  • one made using ytterbium
  • the third using strontium

To take their measurements, the research team set up the aluminium-ion and ytterbium clocks at a National Institute of Standards and Technology lab in Boulder and the strontium clock roughly a mile away at the University of Colorado’s JILA lab.

The idea is to measure how transmitting measurement data between these distances would impact its accuracy. The data was transmitted using both a 2.2-mile long optical fibre and a .9-mile stretch of free-space link communication via laser pulses.

Over several months the team pinged this atomic data back and forth between the institutions (stopping only briefly for a snowstorm) to determine how reproducible and accurate their measurements were. The goal was not to choose the best element for a new atomic clock, but to instead perfect the ways these elements’ time-keeping accuracy was compared. With those new standards established, it would then be possible to find a replacement for cesium.

From their experiments, the research team was able to make the most accurate measurement to date of these ratios between the clocks and also determined that the free-space link provided the same level of uncertainty as the longer, bulkier optical fibre.

“The authors’ demonstration that high accuracy clocks can be connected by free-space links, without needing an optical-fibre infrastructure, is exciting because it opens up possibilities for applications outside the laboratory, such as land surveying,” says Godun.

What’s next — This new research has not yet shaken the longstanding definition of a second, but it has made serious progress toward ushering in a new era of atomic time-keeping. Continuing to refine and test these models could one day soon transform the meaning of a second — improving not only international timekeeping but boosting the accuracy of everything from self-driving cars to your FitBit as well via GPS.

Abstract: Atomic clocks are vital in a wide array of technologies and experiments, including tests of fundamental physics. Clocks operating at optical frequencies have now demonstrated fractional stability and reproducibility at the 10^−18 level, two orders of magnitude beyond their microwave predecessors. Frequency ratio measurements between optical clocks are the basis for many of the applications that take advantage of this remarkable precision. However, the highest reported accuracy for frequency ratio measurements has remained largely unchanged for more than a decade. Here we operate a network of optical clocks based on 27Al+ (ref. 6), 87Sr (ref. 7) and 171Yb (ref. 8), and measure their frequency ratios with fractional uncertainties at or below 8 × 10^−18. Exploiting this precision, we derive improved constraints on the potential coupling of ultralight bosonic dark matter to standard model fields. Our optical clock network utilizes not just optical fibre, but also a 1.5-kilometre free-space link. This advance in frequency ratio measurements lays the groundwork for future networks of mobile, airborne and remote optical clocks that will be used to test physical laws, perform relativistic geodesy and substantially improve international timekeeping.