Morten Bakker (PhD thesis defense June 17): "A new building block for the quantum computer"


(By: Leiden University Communications)

The race to build the first quantum computer is still ongoing, but Morten Bakker has made big step forward in that process with qubits. A qubit is a unit of quantum information that can be produced in large numbers on chips. Qubits capable of exchanging photons (light particles) could be used in the future as building blocks for a quantum computer.

Micro towers

For several years, Morten Bakker could be found on the ninth floor of the Huygens laboratory, where he has been testing chips that could serve as the basis for the future quantum computer. Ordinary chips are usually made from the semi conductive silicon, but Bakker’s experimental chips revolve around layers of two other semiconductors: indium arsenide and gallium arsenide. These chips – though the researchers themselves talk about samples – are especially made in Santa Barbara, California, where a part of the research group of Bakker’s supervisor Dirk Bouwmeester is located. By building a pattern of micro towers on one of these samples and cooling it down to five degrees above the absolute zero (minus 268 degrees Celsius), each of these towers starts functioning as a single qubit.

Only a hundred qubits

The classic computer, that all of us have on our desks and carry in our pockets, relies on a network of billions of electronic switches known as bits. These switch between the on and off position at lighting speeds (being either a 1 or a 0). While it might sound a bit strange, a qubit is a special bit capable of being in the 1 and 0 position at the same time. This enables a quantum computer to do billions of calculations simultaneously, and just about a hundred qubits will be enough to easy outperform a regular computer.

A virtual atom between a bunch of mirrors

Through their PhD research, Bakker and Martin van Exter have been able to make this type of qubit practically usable. The core of these special bits consists of a quantum dot, a virtual atom that can simultaneously operate in two energy states (the zero and the one). Bakker: ‘It is essentially a cluster of a few thousand atoms with one extra electron locked inside.’ Around that, in the material itself, a hollow area can be found that is comparable to two hollow mirrors pressed against each other. The virtual atom can simultaneously operate in a ‘reading’ and ‘writing’ stage thanks to an infrared laser that bounces around the atom about two thousand times inside that structure. Thanks to that hollow structure a single photon is already enough for it to work, though more would make it impossible to retain the mysterious quantum character of the bit.

The chip with the sample. It consists of 42 microstructures, each with its own hollow area and quantum dot, and an additional electron that has been locked in and behaves like a virtual atom. This entire sample is found in the centre of the tower, which has the same diameter as a human hair.

Good samples are rare to find

Making these samples is difficult, as the characteristics of the hollow structure and the atom need to perfectly align. Getting a perfect sample is like winning the lottery, but fortunately Bakker didn’t need too many: ‘Once they’re cooled, they don’t wear out or anything. We used to have a very good sample that was able to work continuously for more than a year, but unfortunately the building was struck by lightning. The cooling cell lost power, which caused the characteristics of the sample to change. We tried cooling the setup again, but it was too late.’

Exchanging quantum information

Thanks to Bakker’s research, the ‘reading’ and ‘writing’ of separate qubits using individual infrared photons has now become fairly easy to realize. The next step, and an ambitious one, is to enable these type of qubits to exchange quantum information among themselves through photons. If they succeed, the quantum computer will have basically become a reality. However, that will be a project for Bakker’s successors, as he intends to continue his career outside the university, as researcher at a company like ASML or Philips.