Scientists from the Council for Scientific and Industrial Research’s (CSIR’s) National Laser Centre (NLC) will soon be trapping ionised atoms of ytterbium (Yb) in a so-called radio frequency Paul trap, cooling the atoms with lasers and using them to measure tiny effects, such as the force exerted by weak electric fields, and to study quantum technologies, says CSIR NLC senior researcher Dr Hermann Uys.
This may enable the scientists to develop technologies for applications ranging from navigation to material characterisation and computational machines that can far outdo modern computers in certain calculations.
The CSIR is building a laboratory for this purpose, which is expected to begin conducting research in 2012. The trap uses radio frequency electric fields to capture atoms.
“With the correct vacuum conditions, a single atom can be captured for an extended period, even up to a month. The scientists give the atoms pet names because they are captured for such a long time,” he quips.
These experiments can only be done with the right laser technology. In this case, four lasers are needed for the purposes of ionising, cooling, manipulating and detecting the electronic internal state of the atom, he says.
Uys explains that the temperature of a trapped atom refers to the average velocity distribution of the atom relative to the ion trap. The hotter an atom is, the wider its velocity distribution, and vice versa. With the right techniques, an atom in such an experiment can be cooled down to submillikelvin temperatures, or ten-thousandths of a degree Celsius above absolute zero, close to the atom’s quantum ground state.
One standard cooling technique is known as Doppler cooling and relies on the prin- ciple that, when an atom moves towards a light source, the frequency of light that the atom experiences is somewhat higher, analogous to the higher pitched sound an observer hears from an approaching Formula One car. Conversely, the atom experiences a lower frequency when it moves away from a light source, just as one would hear a lower pitched sound from a Formula One car driving away.
“Light can impart momentum to the atom by scattering off the atom through a process called fluorescence. The atom will be cooled if a setup can be arranged that imparts momentum in such a way that it slows the atom down, that is, narrowing its velocity distribution. And that is where the Doppler effect comes in,” he says.
The atom will fluoresce only at very specific frequencies of the light. The frequency of the laser beam is chosen so that, when the atom moves towards the laser beam (counterpropagates) and experiences a higher frequency, it will fluoresce more strongly, but less strongly when it moves away (copropagates) and experiences a lower frequency.
The average momentum transfer is always in the direction of the propagation of the laser, so, in this setup, a counterpropagating atom will be slowed down, while copropagating atoms will speed up. However, owing to the fact that a counterpropagating atom scatters light more strongly, on average, the atom is slowed down and, consequently, it cools down, he explains.
Once cooled, the atom can be used as an extremely sensitive measurement probe. For example, a single trapped ion is sensitive to forces down to the yoctonewton force scale, notes Uys.
One might be able to sense tiny electric fields above the surface of specialised materials with such a probe. Ion traps also hold the record for the most accurate atomic clocks to date, although they are having a neck-and-neck race with neutral atom clocks, he says.
Another use for the system is for studies in quantum mechanics, with many researchers working to develop a quantum computer. Many of the basic processes needed to build a quantum computer have been demonstrated using ion trap systems, says Uys.
“Such a computer would be able to carry out certain tasks much faster than conventional computers, such as factorising large prime numbers, a function of interest to cryptographers,” he adds.
A related field of study is quantum simulation, in which the atoms are coerced to behave as though they are rather the particles of a completely different material. For example, the Yb atoms can be made to resemble the magnetic moments in a magnetic material and then tests can be carried out that simulate the study of the actual magnetic material and how such a system would work, he explains.
The new ion trap being built at the CSIR NLC will be one of three cold atom systems in South Africa, with the other two capturing neutral atoms of rubidium at the universities of Stellenbosch, under Christine Steenkamp, and KwaZulu-Natal, under Francesco Petruccione, already in operation.
“There are only a few groups in South Africa conducting studies of quantum mechanics using cold atoms and this is an exciting time for the field here,” he enthuses.
“Physics is an exciting field that evolves at a dazzling pace. There is always something new to discover and it is always worth studying these new phenomena. I encourage any student interested in the natural sciences to consider physics as a career,” he concludes.