Delft researchers take a step forward with their quantum computer

Silicon-based building blocks for quantum computers have reached an important milestone. Three groups of researchers – including one from Delft – have shown that it can be used to perform operations with sufficient accuracy for reliable results. This makes these building blocks a serious candidate for future quantum computers.

Quantum computers promise to perform certain tasks much faster than today’s computers and to solve problems that are currently beyond our reach. That power comes from building blocks called qubits, which are not only one or zero – like the ‘regular’ bits – but also a combination of them. They can consist of different particles: electrons, photons or superconducting circuits.

Electron qubits trapped in silicon structures are a good candidate because they are small and stable. In addition, production can benefit from the existing chips industry. One drawback was that silicon qubits were more likely to make mistakes.

That problem now seems to be solved. Researchers from QuTech, a collaboration between TU Delft and TNO, demonstrate operations with two qubits with 99 percent reliability. “Silicon qubits are therefore no longer inferior to other candidates,” says quantum physicist Xiao Xue of QuTech. The results appeared in Nature on Wednesday.

Sometimes things go wrong

To perform calculations with qubits you have to perform operations with them, but that sometimes goes wrong. These errors can be fixed, but only if not too many errors occur. “Therefore, a confidence limit of 99 percent has been determined,” says Xue. “A qubit technology is only taken seriously if it is above 99 percent.”

The Delft researchers managed to perform operations with two qubits with more than 99.5 percent reliability. They have improved the quality of the silicon, so that the sensitive qubits are less likely to be disturbed. “We have also improved the control and calibration of the system,” says Xue.

Two other publications with comparable results appeared in Nature. Japanese researchers also used the improved material from Delft. And Australians had a slightly different technique: not with electrons, but with phosphorus atoms in silicon.

“It is a nice, impressive step,” says physicist Carlo Beenakker of Leiden University, who was not involved in the research. “It’s a start. You can’t do much with two qubits, the challenge is to make hundreds that are just as good.”

You need at least a million qubits for useful applications. Silicon qubits seem to be lagging behind. Google has a quantum computer with dozens of superconducting qubits with more than 99 percent reliability. But those qubits are much larger and further scaling is a challenge. The promise of silicon qubits is that you can scale them more easily. Beenakker: “If silicon qubits live up to their promise and prove reliable on a large scale, then Delft has bet on the right horse.”