Novel microcavities can store light beams for split seconds and allow them to interact with each other – this is an important step towards building quantum simulators.
Even a supposedly simple piece of matter can possess incredibly complex quantum physical properties. In the interest of developing new materials or improving existing ones, researchers want to better understand these quantum properties. Unfortunately, they are often impossible to observe at all, sometimes they are even impossible to simulate on a computer without tremendous difficulty. One approach is to use devices called quantum simulators instead to simulate the elusive quantum properties and interactions. One promising technology for such a quantum simulator is given by optoelectronic chips. By cleverly adjusting the physical parameters of these chips, they can be made to behave like other solid state systems that are of interest but are much more difficult to control, thereby simulating them.
The key components of optoelectronic chips are tiny cavities where light and electrons can interact with each other. Such microcavities already make good light emitters by themselves, serving as micro- or nanolasers for example. So far, however, it has proven difficult to produce complex structures small enough to exhibit characteristic quantum behaviour. A research team from the Paul Drude Institute (PDI) in Berlin has now made an important step towards this goal. Using molecular beam epitaxy, they were able to shrink down the size of the most important structures.
The structures we are talking about are called “exciton-polaritons”. This term in solid state physics refers to coupled states of light and electrons inside matter. If a light pulse encounters the right kind of microstructure, it can be stored there for a short period of time. To make this happen, two effectively perfect mirrors have to be placed opposite of each other at a spacing of a few hundred nanometres (billionths of a metre) so that the light pulse forms a standing wave inside the tiny cavity. This wave then causes the electrons in the material to start vibrating as well, until the light and electrons harmonize into a coupled state with distinctive properties. These can be exploited to produce laser beams, for example, or to emit light with other useful spectral properties. The new technology now makes it possible to produce these kinds of exciton-polaritons on a scale of one micron (millionth of a metre). The smallest previously achieved were many microns wide – and thus too large for applications in micro and quantum technology.
The researchers also want to manipulate the properties of the cavities using a special trick. “Like in an ultrasound device, we want to get the structure vibrating at extremely high frequency,” Alexander Kuznetsov explains. At a frequency of several gigahertz – about a million times higher than the limit of human hearing – standing waves form on the material with all their peaks and troughs arranged very closely together. This allows one to produce a time-variable structure in the material, which in turn allows further control by tweaking the interactions of the cavities with the light and electron vibrations.
The new technology from the PDI researchers is an important step towards quantum simulators, and the miniaturisation they have achieved is sorely needed in the development of new concepts for quantum simulators. “Except, we are not quite there yet,” Kuznetsov cautions. The structures still have to be shrunk to a scale of about half a micron for it to work. So, the elements are still just a little bit too large. However, the scientists have already identified the various mechanisms that play a role in producing structures this small.
For the next stage, the researchers plan to use their optoelectronic chips to study how the microcavities behave when the solid body is simultaneously brought into high frequency vibration. A further aim for the research project is to produce even smaller components and quantum simulators. These are being developed, among other efforts, within the European cofunded project “InterPol”, in which the PDI is involved.
Text: Dirk Eidemüller
