Millions of qubits are required for quantum computers to prove useful in practical applications. Scalability is one of the biggest challenges in developing future devices. One problem is that the qubits have to be very close to each other on the chip for them to be fused together. Researchers at Forschungszentrum Jülich and RWTH Aachen University have now taken an important step toward solving the problem. They successfully transmit electrons, the carrier of quantum information, over several micrometers on a quantum chip. Their “quantum vector” could be the key ingredient to mastering the leap into millions of qubits.
Quantum computers have the potential to vastly exceed the capabilities of conventional computers at certain tasks. But there is still a long way to go before they can help solve real-world problems. Many applications require quantum processors with millions of qubits. Today’s prototypes come with just a few of these computing units.
“Currently, each individual qubit is communicated via several signal lines to control cabinet-sized modules. This still works with a small number of bits. But it no longer makes sense if you want to put millions of qubits on a chip. Because that is necessary for quantum error correction,” he says. Dr. Lars Schreiber of the Jära Institute for Quantum Information in Fürschengcentrum Jülich and RWTH Aachen University says.
At some point, the number of signal lines becomes a bottleneck. Fonts take up a lot of space compared to the size of the small bits. Nor can a quantum chip have millions of inputs and outputs – a modern classic chip has only about 2,000 of them. Together with colleagues at Forschungszentrum Jülich and RWTH Aachen University, Schreiber has conducted research for several years to find a solution to this problem.
Their overall goal is to integrate parts of the control electronics directly onto the chip. This approach is based on the so-called spin qubits of semiconductors made of silicon and germanium. This type of qubit is relatively small. The manufacturing processes are largely identical to traditional silicon processors. This is useful when it comes to achieving a very large number of qubits. But first, some basic barriers must be overcome.
“The natural entanglement caused by particle proximity alone is limited to a very small scale, around 100 nanometers. To pair the qubits, they must currently be placed very close to each other. There is simply no room for more,” Schreiber says.
To distinguish qubits from each other, the Jara Institute of Quantum Information (IQI) came up with the idea of a quantum shuttle. This special component should help exchange quantum information between qubits over larger distances. Researchers have been working on the Quantum Vector for five years and have already filed more than 10 patents. The research began as part of the European QuantERA Si-QuBus Consortium and is now continuing on the QUASAR National Project of the Federal Ministry of Education and Research (BMBF) together with industrial partners.
“About 10 micrometers must be bridged from one qubit to another. According to the theory, millions of qubits can be achieved using such an architecture. We recently anticipated this in collaboration with circuit engineers from the Central Institute for Engineering, Electronics and Analytics at Forschungszentrum Jülich,” explains Professor Hendrik Blum, Director of IQI . Researchers at TU Delft and Intel also came to this same conclusion.
Lars Schreiber and his team have now made an important step. They succeeded in transferring an electron 5,000 times at a distance of 560 nanometers without any major errors. This corresponds to a distance of 2.8 mm. The results were published in the scientific journal Quantum information npj.
One key improvement: The electrons are powered by four simple control signals, which – unlike previous methods – don’t get any more complicated over longer distances. This is important because otherwise extensive control electronics would be required, which would take up a lot of space – or could not be integrated on the chip at all.
This achievement depends on a new method of electron transfer. “Until now, people have tried to guide electrons specifically around individual perturbations in their path. Or they have created a series of so-called quantum dots and let electrons travel from one of these dots to another. Both approaches require subtle signal modulation, which results in very complex control electronics. Lars Schreiber explains. “In contrast, we generate a potential wave in which the electrons simply navigate through various sources of interference. Some control signals are sufficient for such a uniform wave; all it takes are four sine pulses.”
As a next step, the physicists now want to show that the qubit information encoded in the electron spin is not lost during transmission. Theoretical calculations have already shown that this is possible in silicon at certain speed ranges. Thus the quantum carrier paves the way for a scalable quantum computer architecture that could also serve as the basis for several million qubits.