An open race
One way to create qubits is with semiconductors. The team led by Prof. Hendrik Bluhm from the JARA Institute for Quantum Information uses two single electrons trapped in a tiny wafer in a kind of trap. They form the qubit. The wafer consists of five semiconducting layers that differ slightly in composition but mainly contain the elements gallium, arsenic and aluminium. The JARA scientists at Jülich’s Helmholtz Nano Facility have applied a tiny grid-like structure made of metal to the top layer.
Due to the special layer structure, a plane exists in the wafer on which electrons move back and forth without being able to leave the plane. The scientists can restrict the remaining free space of the electrons in the plane by applying an electrical voltage to the metal grid. This is because the negative charge on the metal forms an insurmountable hurdle for the electrons, which are also negatively charged: the trap called the quantum dot has snapped shut.
The JARA researchers’ semiconducting qubit wafers are particularly closely related to the chips used in today’s microelectronics. As the industry has gained extensive experience with the production of microchips for decades, semiconductor qubits could be particularly suitable for future upscaling.
However, there are also other approaches to creating qubits. It is still open as to which technology will win the race in the end. At Jülich, scientists are also researching superconducting qubits. This approach is currently considered leading, with Google and IBM, for example, relying on it. Here, qubits are generated from currents that flow without resistance in superconducting circuits. It is still unclear whether computers with thousands of these qubits can be developed.
While functioning systems have already been developed for semiconductor and superconducting qubits, research into hybrid qubits is still in its infancy. The idea: a so-called topological insulator is applied to an ordinary superconductor. Jülich is researching this, and so is Microsoft. Topological insulators are a class of materials that, in simple terms, have the properties of an insulator on the inside and those of a conductor on the outside. This would make it possible, at least theoretically, to create a qubit that is less susceptible to interference than, say, semiconducting or superconducting qubits, which are very sensitive. Even the smallest disturbances can cause errors.
Candidates for hybrid qubits are Majorana particles, which are, however, difficult to generate, and so-called gatemons, in which a superconducting qubit is modified by a topological insulator.
Illustration: Andrzej Koston