The race to be the first to develop a functional quantum computer is going on in both industry and academia. For example, both Google and Chalmers University of Technology are developing their own quantum computers that will be able to perform advanced calculations that today’s traditional supercomputers cannot handle.
It is hoped that quantum computers will revolutionise several different scientific fields such as pharmaceutical production, materials development and give a deeper understanding of how the laws of nature work.
“If we are to be able to supplement ordinary computers with quantum computers in the long term, they need to work at room temperature. We are investigating which materials could be suitable for this,” says Rickard Armiento, researcher in theoretical physics at Linköping University and examiner of Oscar Groppfeldt’s master’s thesis.
Qbits handle the information
There are essentially three different principles on which quantum computers can be based – superconducting circuits, quantum points in semiconductors and material defects. To describe how the three principles actually work would take some time. However, they share some basic features.
A quantum computer works with something called quantum bits, or qbits, to handle information. You could compare qbits with ones and zeros as in a normal computer. What differs is that a qbit does not have to be in one or the other position, but can be in any position between one and zero. This is called superposition and opens up the possibility to handle much more information in less space.
The quantum bit, when we talk about material defects, is actually an electron that is located between two energy levels in an atom, that is, in superposition. When an electron picks up energy from light (a photon), it jumps up an energy level. But the electron will inevitably return to its original state, and then it emits a photon. This optical process is used to process information in a quantum computer. By manipulating the electron’s energy levels in the material, it can get into superposition, thereby increasing the amount of information it can handle.
The Y programme
What Oscar Groppfeldt has done in his master’s thesis on the MSc programme in Applied Physics and Electrical Engineering (the Y programme) is to investigate which material defects could be suitable for future quantum computers. Currently, the most common material defect to depart from is a diamond where two carbon atoms are removed and a nitrogen atom added instead. This is called an NV centre. But there are many other options.
“Imagine that we have a crystal in which the atoms repeat themselves in a space, and then you remove something, like a nitrogen atom. Then what we call a defect arises that can give a lot of interesting states, which you can manipulate and get information out of. But then you need to have a defect that makes sense,” says Oscar Groppfeldt.
You may think that a defect indicates the material is damaged and therefore a randomness arises. But in this case, the opposite is true. These are very precise changes in the structure of the material that the researchers want to take advantage of.
Material defects
To find a “sensible defect”, Oscar Groppfeldt has used a list of possible candidates and run simulations on them, using supercomputers.
“I’ve run through a total of 13 materials with about 4,000 defects. I ran into problems with some of the materials behaving differently in different directions. So I had to write some new code to cover this type of material behaviour,” says Oscar Groppfeldt.
What he has done is to use computer calculations to simulate the electrons in a variety of materials where he systematically replaced and removed atoms to see which optical properties can be used for quantum bits.
Supercomputers are key
According to Joel Davidsson, postdoc at the Department of Physics, Chemistry and Biology and thesis supervisor for Oscar Groppfeldt, the work would not have been possible just 10 to 15 years ago. It is all down to the development of supercomputing power.
“It would take a lot of work just to realise some of these systems in a lab. Using the supercomputers, we run through all possible options and look for defects in materials that we can manipulate in the right way,” says Joel Davidsson.
The results of the thesis are published in a large database of material defects that is run at LiU and is openly accessible via the Internet to other researchers and companies who want to continue the development of quantum computers. Oscar Groppfeldt continues to look for more potential defects, but now as a doctoral student at the Division of Theoretical Physics at LiU.
“The degree project was fun! I haven’t really worked that much with material physics before, but it was exciting and fun to learn something new. It’s more fun to do something that’s challenging than just take the easy way out.”
The material defect database: ADAQ Database
