The World’s Time

Andrei Derevianko

Quantum clock
Credit: Jun Ye group and Steve Burrows, JILA

Futuristic quantum clock network.

Atomic clocks
Credit: PTB

A collection of atomic clocks at the Physikalisch-Technische Bundesanstalt (PTB), Germany. These clocks substantially contribute to the TAI timescale, the world’s time.

As I type this text away, I become aware that time just continues its quiet flow and my keyboard clicks measure its passage. And whatever poetical, philosophical or religious meaning one might assign to the “time”, that is how the time is defined: as a measurable sequence of events.

And time being measurable naturally means that physicists are in business.

Atomic clocks are arguably the most accurate devices ever built. While a typical wristwatch keeps time accurate to about a second over a week, modern atomic clocks aim at neither gaining nor loosing a second over the age of the Universe. Imagine that if some poor soul were to build a clock like that at the beginning of time, at the Big Bang, and for some good reason it were to survive through all the cosmic cataclysms, today it would be off by less than a heartbeat.

Atomic clocks are ubiquitous and one could buy a slightly used one on the internet. Among many places, they tick away on stock exchanges, in data centers, and in the hearts of GPS satellites. However, there is a truly special collection of several hundred atomic clocks distributed among 50 or so industrialized countries that defines the world’s time. This timescale is known as the TAI (from the French “Temps Atomique International”) or the international atomic time.

BIPM (Bureau International Des Poids Et Mesures) is at the heart of defining the world’s time. This international organization is located in a white wooden two-story building on the forested bank of the Seine River in the Parisian suburbs. Judah Levine from NIST-Boulder explains that BIPM was established in 1875 by the international “Treaty of the Meter” which defined the kilogram and the meter. Later the second was added to the convention (SI units) and the meter redefined in terms of the fixed speed of light and the second. The modern legal definition of the second involves a certain number of beats derived from the hyperfine splitting of cesium-133 atom.

Judah Levine has been contributing US data to TAI for nearly half a century. He explains that BIPM collects clock data from metrology labs and averages them. Then BIPM distributes a document called “the Circular T” which tells by how much the national timescales were off from the average about a month ago. In turn, based on this circular, national labs steer their local timescales to account for the drifts from the TAI. Such a protocol keeps the world’s time stable at the level of a nanosecond over a month.

The most advanced metrology labs rely on the so-called primary frequency standards, super-precise cesium clocks, says Peter Rosenbusch of the Laboratoire Nationale de Metrologie et d’Essais and the Paris Observatory. The primary standars are occasionally used to calibrate other local “workhorse” continuously-run atomic clocks to the SI definition of time as close as possible. In the US, the primary frequency standard is the cesium fountain clock at NIST-Boulder.

So if the world’s time is the time counted by atomic clocks, is it the same as the cosmic time? In principle one could measure time using pulsars, magnetized rotating neutron stars. The pulsars, however tend to slow down over time due to the gravitational wave radiation, and, moreover, Judah Levine points out that the very shape of the pulses also changes over time making counting the pulses imprecise. We joke that, perhaps, to define the Standard Galactic Time one needs to find more stable cosmic sources.

Nevertheless, space and satellite technology are anticipated to improve TAI. Christophe Salomon of Ecole Normale Supérieure in Paris is involved with the ACES (Atomic Clock Ensemble in Space) mission of the European Space Agency. He explains that the goal is to operate the most precise primary Cs frequency standard onboard the International Space Station (ISS). The clock is expected to become operational in space in two years. ISS would broadcast a microwave time signal down to several Earth-based stations. In the USA, the stations will be installed at JPL in Pasadena and NIST-Boulder. Through the ACES mission, national labs around the globe will establish high precision links to compare primary standards and thus remove some uncertainties in their contributions to the world’s time.

Neither time nor its definition is still. There are new generations of atomic clocks based on ultracold atoms and ions that already outperform the primary frequency standards. Pushing these quantum devices to their limits is a friendly competition between several labs around the world. Just over the past year the crown of being the world’s most precise clock has been shared by USA (two teams at JILA and NIST-Boulder), Japan, and Germany. These advances have been summarized in recent talks by E. Peik (PTB, Germany) and A. Ludlow (NIST-Boulder, USA) at the International Conference on Atomic Physics held last July in a historic Mayflower hotel in Washington D.C.

Considering this rapid progress in atomic horology, the international community discusses how to redefine the second in terms of these novel classes of clocks. This means retiring Cs from the SI units and redefining the world’s time.

Also the clock comparison technology improves. The European Union is building a trans-European clock network using existing optical fiber communication links to compare clocks at metrology labs directly, removing the uncertainties of the over-the-air and over-the-space comparisons. The first 920 km-long link between the northern and southern parts of Germany has been already tested.

One of the apparent limitations of the TAI timescale is that it is a “paper timescale” – it only shows what the world’s time was a month ago. What if the dedicated clocks were compared and averaged continuously or even better they formed one single geographically distributed clock? This was envisioned recently by a group of physicists led by Mikhail Lukin at Harvard and Jun Ye at JILA in Colorado. They proposed a quantum network of atomic clocks (for example, placed on satellites orbiting the Earth) that would utilize quantum entanglement to create one giant distributed clock with each nation contributing satellites to the network. Jun Ye comments, “this is definitely a futuristic proposal, and we must achieve substantial technological advances. However, all of the different building blocks for the network have in principle been demonstrated in small scales.” May be this is how the world’s time would be measured in the far future.

Dr. Andrei Derevianko is a Russian-American theoretical physicist and a professor at the University of Nevada, Reno. He has contributed to the development of several novel classes of atomic clocks and precision tests of fundamental symmetries with atoms and molecules. Email:

Disclaimer - The articles and opinion pieces found in this issue of the APS Forum on International Physics Newsletter are not peer refereed and represent solely the views of the authors and not necessarily the views of the APS.