“Quantum bus” innovation potentially makes quantum computers scalable

Experts from Forschungszentrum Jülich and RWTH University of Aachen have developed a “quantum bus” that could improve the capabilities of quantum computers, making them scalable.

For quantum computers to be effective in practical applications, they require millions of quantum bits (qubits), which limits their scalability. This is because the qubits must be located extremely close to each other on the chip to pair them together – a significant problem with the technology – which the team may have solved.

Using their quantum bus, the researchers devised a way to transfer electrons, carriers of quantum information, over several micrometers onto a quantum chip. Their innovation could be the essential component to facilitate the leap to millions of qubits.

Current limitations of quantum computers

Quantum computers have the potential to significantly surpass the capabilities of conventional computers for various tasks. However, hurdles persist with the technology, as many applications require quantum processors with millions of quantum bits, with current prototypes only able to muster a few.

Dr Lars Schreiber, from the JARA Institute for Quantum Information at Forschungszentrum Jülich and RWTH University Aachen, explained: “Currently, each individual qubit is connected via multiple signal lines to cabinet-sized control units. It still works for a few qubits. But that doesn’t make sense anymore if you want to put millions of qubits on the chip. Because it’s needed for quantum error correction.

The number of signal lines eventually becomes a bottleneck because they take up too much space relative to the size of tiny qubits. Also, a quantum chip cannot have millions of inputs and outputs; a typical chip contains only about 2000.

To overcome this, the team is researching how to integrate parts of the control electronics directly on-chip using silicon and germanium semiconductor spin qubits. This type of qubit is extremely small, and the manufacturing process is similar to that of traditional silicon processors, although some obstacles remain.

Schreiber said: “The natural entanglement caused by the proximity of the particles alone is limited to a very small range, around 100 nanometers. To pair the qubits, they currently need to be placed very close to each other. There is simply no room for the additional control electronics that we would like to install there. »

Develop a quantum bus

Over the past five years, the team has developed a way to tell qubits apart, designing a quantum bus that allows quantum information to be exchanged between qubits over greater distances.

Professor Hendrik Bluhm, Director of the IQI Institute, said: “About 10 micrometers need to be bridged from one qubit to another. According to the theory, millions of qubits can be realized with such an architecture.

The team’s technology managed to transport an electron 5,000 times over a distance of 560 nanometers without significant errors. The advantage of this method is that the electrons are driven by four simple control signals which, unlike previous efforts, do not become more complex over long distances. This eliminates the need for additional control electronics that would take up too much space or could not be integrated on the chip at all.

Schreiber said, “Until now people have tried to direct electrons specifically around individual disturbances in their path. Or they created a series of so-called quantum dots and let electrons jump from one dot to another. Both approaches require fine tuning of the signal, resulting in overly complex control electronics.

“In contrast, we generate a potential wave on which the electrons surf on various sources of interference. A few control signals are sufficient for such a uniform wave; four sinusoidal pulses are sufficient.

The quantum bus is leading the way to scalable quantum computers, and researchers are now working to show that the qubit information encoded in the electron spin is not lost during the transport process.