HIGH PRECISION EXPERIMENT
The hunt for a single ion
It’s the GPS system that’s a bit off kilter. It uses the signal of at least three satellites to determine your position on the globe. However, all too often the atomic clocks used by the satellites are not accurate enough. Theoretically it’s possible to determine your position far more precisely, in tenths of centimetres even. However, to put theory into practice, first you have to catch a single ion, preferably Radium.
That’s exactly what a group of scientists led by Professor Klaus Jungmann and Associate Professor Lorenz Willmann are trying to do. This summer they took the first step when their team succeeded in catching a single Barium ion.
Why? What do ions have to do with your GPS?
To understand that, you need to realize that GPS works by measuring the time it takes the GPS signal to travel from a satellite in space to your car. At the moment Cesium clocks are used to define our time. Their accuracy is impressive: 1014 to be exact, which means, according to Jungmann, that ‘in one year it’s off by a tenth of microsecond’. That’s not something you would notice in your daily life, not even over a lifetime, but scientists aim for higher precision. ‘The clock that we are building will exploit a single Radium ion. It has an accuracy of 1018 and will be able to determine your position in tenths of centimetres’, Jungmann explains.
Jumping back and forth
The team started building their lab four years ago, in an old office, where they first had to clear out the desks and paint the walls. Now Jungmann and Willmann have assembled two rooms full of equipment worth hundreds of thousands of euros and a team comprising two technicians and three PhD students. One of the PhD students is Mayerlin Nuñez Portela, from Colombia. ‘This project combined my previous experience from my Master’s research with physics. Constructing a high-precision experiment seemed like a challenge’.
The principle of the atomic clock is simple: it works just like a grandfather clock. The pendulum repeats a motion and a system of cogwheels and radars translates the movement into time.
However, it doesn’t take the same amount of time for the pendulum to swing back and forth every time. That is influenced by the resistance from air molecules or by you bumping into it.
These influences can be reduced when the motion is repeated extremely fast and that’s the situation you can find in an ion, where the ion travels in discrete shells around the nucleus of the atom. To get from one shell to the other, the electron absorbs energy in the form of light provided by a laser, but when it jumps to the next shell, it immediately loses its energy again and falls back to the first shell. Since this jumping back and forth between shells happens extremely fast, the process is very useful for creating a highly accurate clock.
Invisible to the naked eye
So, catch an ion, you might say, and create your clock. However, it’s not that simple. ‘For the Cesium clock measurement, the atoms aren’t floating freely. There are millions of atoms, all influencing each other, but if an atom is not influenced by anything, it should always behave the same way and be more accurate. So we have to trap that one single ion’.
However, trapping something so tiny that it is invisible to the naked eye in a controlled environment is not that easy. Nuñez Portela remembers her first year. ‘When I arrived in 2010, our first set-up was not working. We had to take it apart piece by piece and check what was wrong’. Months were spent doing this before the team finally came up with a working experiment. However, when they took their first measurements, they realized almost immediately that catching that single ion would not be possible with the set-up they had. They could not even get under 100,000.
Nuñez Portela was not disappointed, though. She knew a task like this would never be easy. ‘You are conducting a high-precision experiment, so you need extreme control over the conditions. Even a single fingerprint in the wrong place can influence the outcome’.
So the team went back to the drawing board and refined their experiment. New equipment was ordered, either handmade or from the open market, but that is often a time-consuming process. ‘Luckily, we are in Europe. You would not have been able to get your hands on it if you were in Tehran or India, for example. Customs officers will not let that type of equipment into their countries. For all they know you could be building a bomb’.
Again, though, there were months of waiting for equipment to be carefully assembled and tested. Nuñez Portela stayed optimistic, however. ‘I always believed that we would get one single ion. There were days that things you did not expect to break broke anyway, but then you fix them and you continue. It is like everything you do in life. It takes many steps and sometimes it does not go according to plan’.
When she turned on the set-up for the first time, small clouds of ions were trapped in the laser beam. That was better than the first time, but not nearly good enough. Nuñez Portela spent days in the lab trying to understand the characteristics of the set-up. ‘It is quite a frustrating and time-consuming process. It is just turning knobs and trying to understand what happens. As a reward, though, we saw the number of ions going down’.
Then the day came when there was only one ion left. Nuñez Portela remembers it very well. ‘It was a Saturday. Some team members came into work because we were so close that week. We had just a few ions left. We had five, then three and then, all of a sudden, there was only one left! We had a barbecue to celebrate in April, in the rain’.
It was a big achievement, yes, but there’s no Nobel Prize on the horizon just yet. Jungmann smiles. ‘The Nobel Prizes for trapping a single particle and clockwork have already been awarded. A single ion was trapped for the first time as long ago as 1977.’
The experiment will be regarded as a real success when the measurement is repeated for a single Radium ion, as this has never been done before. However, this is not a simple step, as Nuñez Portela explains: ‘Radium is radioactive, so you need a permit to work with it. You also need to think really carefully before starting your experiment. You need to make sure that your clock doesn’t become radioactive inside. Otherwise, you won’t be able to work with it for some time.’
Nevertheless, Jungmann is ready. ‘If everything works, then we just monitor the experiment by looking at computer screens. Even though it produces incredibly accurate results, it gets boring after a time. Then it is time to think about the next challenges’.