Can we capture photons

Cages for cold atoms and ions

One of the most successful methods of manipulating individual particles is based on the combination of electromagnetic traps and laser cooling.

A look into a magneto-optical atom trap

There are many types of particle traps: Ions are caught either in alternating electric fields with a Paul trap or with the help of a combination of electric and magnetic fields (Penning trap). Neutral particles can be caught with magnetic fields, focused laser beams (dipole trap) or with the help of a combination of magnetic fields and laser beams (magneto-optical trap, MOT). Ion traps in particular have been used successfully since the 1950s. In 1989, Wolfgang Paul and Hans Dehmelt were awarded the Nobel Prize in Physics for their development.

The final breakthrough came with the development of laser cooling processes for atomic gases and ions in the 1980s (Nobel Prize 1997 for Steven Chu, Claude Cohen-Tannoudji and William D.Phillips). Although it may initially contradict all intuition that you can cool atoms and ions with laser beams, it is possible to cool particles in large numbers to almost absolute zero by selecting the appropriate laser intensity and detuning the laser frequency. At the temperatures reached in the micro-Kelvin range, the atoms do not even move at walking pace. They can therefore be easily captured and observed undisturbed over long periods of time. If the atoms are not held in a trap, then under the influence of gravity they will fall down just like a macroscopic object.

Today's clocks - tomorrow's clocks

A possible and very promising area of ​​application for laser-cooled particles is the construction of extremely precise clocks. The best modern clocks are based on precisely measuring the frequency of the electromagnetic radiation - light or microwaves - that changes the quantum mechanical state of specific atoms, ions or molecules. Such frequencies can be determined the better, the more time one has available for the measurement - and this is where the great advantage of using laser-cooled particles lies: They move much slower than uncooled particles and therefore remain correspondingly longer, maybe seconds instead of Milliseconds, in a measuring apparatus of a given size. A single, laser-cooled ion stored in a trap is ideal in this respect: In principle, it is available for a measurement as long as you like.

The accuracy of atomic clocks has already been substantially improved by laser cooling processes: The latest cesium atomic clocks, which work at a microwave frequency of 9.2 gigahertz (9.2 billion oscillations per second), use a “fountain” of laser-cooled atoms. The atoms cover their parabolic trajectory in about one second. These clocks achieve an accuracy of 3 × 10-15, d. H. they deviate from the exact time by only one second in 100 million years and are thus about ten times more accurate than earlier cesium atomic clocks.

Even more impressive improvements are expected in the next few years. In the meantime, for example, at the MPI for Quantum Optics in Garching, optical frequencies (approx. 1014–1015 Hertz) measured directly or compared with microwave frequencies. Atomic clocks that like the optical transitions in stored ions 199Hg + or 171Utilizing Yb + should have a much higher accuracy due to the many times higher crossover frequencies and the long storage times. The aim is a thousandfold increase in the previously achieved accuracy: Over a period of 10 billion years, i.e. the estimated age of our universe, such a clock would only go wrong by a tenth of a second!

But why do you actually need watches with such impressive accuracy? First of all, there are a number of practical applications such as navigation using GPS (Global Positioning System), the synchronization of communication networks or, perhaps surprisingly, of generators in power plants. However, the greatest demands on the accuracy of clocks are made by experiments on some of the most fundamental questions in physics - for example, whether the laws of nature are really immutable or whether they change with the age of the universe. But also greatly improved tests of the general theory of relativity, which is fundamental for our understanding of the universe, would be possible.

How do you cool with light?

Interaction between atom and light

The physical basis for cooling atoms with laser light goes back to the work of Albert Einstein. He postulated that light can have particle properties and introduced photons as light particles. A photon represents the smallest unit of light energy and, like a particle of matter, has an impulse. This impulse is too small to appear in our everyday world. For a single atom with its small mass that interacts with a photon, this momentum can lead to a considerable change in speed. For example, if a rubidium atom absorbs a photon, its speed changes by about 5 millimeters per second. Similarly, when the atom emits a photon, it experiences a recoil. This emission process usually happens spontaneously and is evenly distributed across all spatial directions. On average, this results in a change in the speed of the atom. Since the absorption and emission processes can take place up to ten million times a second, it is possible to decelerate an atom moving at supersonic speed to less than walking pace in just two hundredths of a second.

Lasers cool down to the lowest temperatures

The atomic movement can be slowed down, i.e. cooling, can be achieved with two opposing laser light fields, the frequency of which is slightly lower than the resonance frequency of the atom to be cooled and which therefore cannot excite the resting atom. If the atom moves towards one of the two laser beams, the frequency of the light increases as a result of the Doppler effect when viewed from the atom, so that it can now absorb photons and is thus slowed down. With three pairs of opposing beams, the movement can be dampened in all three spatial directions, and the atom remains stuck in the light field like in molasses. With this simple cooling scheme, temperatures of a few 100 microkelvins can be achieved. More complicated methods that take into account the internal structure of the atom can fall below this limit. With the help of so-called sub-Doppler cooling, temperatures of a few microkelvins can be achieved. This limit temperature is given by the small photon pulse associated with a single absorption and emission process. However, this limit has already been undercut by deliberately exploiting quantum interference.