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Menno Veldhorst and Nico Hendrickx standing next to the setup hosting the germanium quantum processor. Photo by Marieke de Lorijn for QuTech.

Germanium qubits in a 2×2 setting

Wouldn’t it be convenient if we could use the already existing state-of-the-art production techniques of the semiconductor industry for designing quantum computing devices? That is exactly what researchers at QuTech are doing. And they just reached a milestone in this process.

There are several major challenges that must be addressed before we have a fully functioning quantum computer consisting of roughly a million qubits. Two of these challenges are the issue of scalability and quantum decoherence. Researchers from the group of Menno Veldhorst at QuTech found a way to address both challenges.

First the issue of scalability. We can build one qubit, maybe even set-up another one that can communicate with its neighbor, but adding a third one is already quite difficult. Hendrickx (first author of the publication in Nature) and his colleagues managed to set up four qubits that could successfully communicate with each other. A key detail here is that the four qubits are set-up in a 2×2 square, and not in a line. The qubits can be successfully entangled and controlled at will. This 2-dimensional set-up is highly beneficial for the scalability of quantum processors.

Next comes the issue of decoherence. Qubits are fragile and can easily lose their quantum state through interactions with the environment. A material that successfully hosted two qubits without them being affected by noise too much was the most renowned semiconductor, the material that Silicon Valley owes its name to. However, scaling with silicon beyond two qubits has remained a challenge. Researchers therefore started to explore other materials and it turns out that an excellent host for qubits is the material behind the very first transistor: Germanium.

The researchers used quantum dots as qubits. Quantum dots are structures very similar to the transistor, but are able to capture individual electrons. Quantum dots in germanium can also capture holes. A hole is a missing electron, which is realized by applying a negative voltage on the quantum dot. Holes can be highly efficiently controlled as a qubit and can maintain coherence for a long time, in particular when defined in germanium. And last-but-not-least, Germanium is a standard material used in advanced semiconductor manufacturing and thus can be easily adopted by semiconductor industry.

The next challenge that awaits is the wiring for the qubits. Right now, every qubit has its own individual wire to control the voltage of every qubit. When we consider a million qubits, we don’t want a million wires. Therefore, the goal of the researchers is to look at addressing multiple qubits at once with one single wire.

For now, the achievements of a 2×2 qubit processor already bring us closer to benefitting from the advancements made in the semiconductor industry. After successfully creating the first germanium quantum dot qubit in 2019, the researchers have doubled the number of qubits on their chips every year.

‘Four qubits by no means makes a universal quantum computer, of course,’ Veldhorst says in an article on QuTech’s website. ‘But by putting the qubits in a two-by-two grid we now know how to control and couple qubits along different directions.’

We look forward to the next doubling of the number of qubits!

You can read the article in Nature here.