RIKEN demonstrates error correction with silicon qubits ― Establishes basic technology toward practical quantum computers

A research team led by Research Scientist Kenta Takeda of the Quantum Functional System Research Group, RIKEN Center for Emergent Matter Science, Special Postdoctoral Researcher Akito Noiri, and Group Director Seigo Tarucha, succeeded for the first time in the world in implementing a three-qubit gate and a basic quantum error correction using spin-based qubits in a silicon quantum dot device. It is the most basic implementation of quantum error correction, one of the challenges in realizing a quantum computer using silicon semiconductors. This achievement will accelerate future research and development. The research result was first published in the online edition of Nature.

Due to their nature, quantum computers are prone to errors, and error correction technology is essential for practical quantum computing. Although basic operations up to two qubits have been realized so far, it has been difficult to fully control the minimum three qubits required for high precision quantum error correction.

The research team has previously realized quantum gates up to two qubits, and now have implemented a three-qubit Toffoli-type quantum gate. A Toffoli gate is an operation that inverts the state of the target qubit (data qubit) only when the two auxiliary qubits are both in the zero state, and in quantum error correction, the state of the data qubit can be corrected based on detected errors.

The group implemented a three-qubit phase error correction circuit using a Toffoli gate. By encoding three qubits into a quantum entangled state, if a phase error occurs in any one of the three qubits, it is detected by reflecting it in the state of the auxiliary qubit through decoding (reverse operation of encoding).

In this research, the group demonstrated that errors could be detected by implementing this quantum circuit on a silicon qubit sample and measuring the state of the auxiliary qubit after decoding. They also demonstrated that the data qubit can be corrected to the state before the error occurred by performing an operation that corrects the state of the data qubit according to the state of the two auxiliary qubits, namely, executing a Toffoli gate. This achievement is a key milestone in the development of quantum computers.