Vibrations and time transfer

Accurately measuring time

A minute with your hand on a hot stove undoubtedly lasts longer than a minute in the arms of a loved one (loosely based on a — disputed — quote attributed to Einstein). In short: time is relative. Yet measuring time with high precision is crucial for many applications, such as the financial sector, telecommunications, and gaining insight into the smallest particles in nature: molecules, atoms and electrons.

That last area is the work of Jeroen Koelemeij, a university lecturer at Vrije Universiteit Amsterdam. “Our understanding of chemistry and biology depends on how well we understand those atoms and molecules. That matters for many areas of society, for example drug development. By looking at how fast atoms and molecules vibrate, we can learn a great deal about their properties and how they function.”

Tapping and probing molecules

How do you make these vibrations visible? Koelemeij traps molecules in a vacuum chamber, where they are fully isolated from external influences such as magnetic fields. By firing a laser beam at them, he sets the molecule vibrating and counts the number of vibrations per second. “We can measure that frequency to as many as twelve decimal places.” Measuring vibrational frequency requires an extremely accurate clock: an atomic clock. Incidentally, it does not look particularly science-fiction-like — it is a flat grey box that mostly resembles a DVD player.

Highly precise atomic clock

“So far, we have been able to do everything using the atomic clock in our own lab, but for the next generation of experiments we want to measure with 14 to 17 decimal places. You can’t simply buy an atomic clock that precise; you have to build it yourself, and that is very expensive.” Every country has its own national metrology institute that maintains the official time standard. In the Netherlands, that is VSL in Delft. “So we needed access to VSL’s atomic clocks, which are 10 to 100 times better than ours. That will allow us to move forward for years to come.”

More stable measurements

To make this possible, Koelemeij uses the SURF Time&Frequency service. An optical fibre cable — as thin as a hair, carrying data at the speed of light — effectively brings the clock in Delft to Amsterdam. “The connection was installed last summer. We immediately saw a major improvement: the measurements are now much more stable, and our researchers are very happy with that. What initially looked like an enormous mountain of six months’ worth of measurements, we can now do in a month.”

The atomic clock signal from VSL takes a fraction of a millisecond to travel the 100 kilometres to Amsterdam. “At the same time, changes in ambient temperature cause the optical fibre in the ground to contract and expand. This creates variations in the signal, making it seem as if the atomic clock in Delft is constantly moving away from us and towards us.” Special White Rabbit network equipment, installed by SURF specifically for this application, measures these variations and corrects them. This makes the measurements far more reliable. The underlying protocol was originally developed and released as open source by CERN in Geneva, and is now used worldwide.

Safer alternative to gps

Time&Frequency transfer has another interesting application, which requires us to zoom out from the atomic scale to the global stage: the technology is more accurate and more secure than the GPS navigation system. So in addition to measuring molecules, Koelemeij also conducts research into Time&Frequency technology itself.

The GPS system consists of satellites orbiting the Earth, each carrying several atomic clocks that periodically transmit radio signals to the ground. GPS receivers on Earth — for example the one in your phone — pick up these signals. Because the satellites are at different distances from your phone, the signals do not arrive at the same time. By calculating the time differences between the signals, your phone determines your location and synchronises its clock with the GPS system. This works with an accuracy of a few nanoseconds (billionths of a second) and a few metres. But it can be even more precise.

“With the SURF network, we can reach fractions of a nanosecond and determine a location down to within 10 centimetres,” Koelemeij explains. “So it is much more accurate.” The fibre-optic connection also offers advantages over GPS. For example, it is not affected by interference from buildings — something GPS signals can suffer from in urban areas, underground, or in shielded laboratories.

Frying satellites

There are also concerns about the security of GPS and the potential consequences of a prolonged GPS outage. “Countless companies and organisations use GPS for navigation and to know the time. But a major solar flare could fry all those satellites, or they could be hacked or disrupted by hostile parties. Then those systems would stop working, and you might no longer be able to make phone calls or pay by card. SURF’s system offers a robust alternative that is less vulnerable to disruption than GPS. In the future, it may well become essential to our society. For me, that is an important reason to do this research.”

Text: Josje Spinhoven
Photos: Vera Duivenvoorden