Phase correction code on spin qubits*

An important requirement for realizing a useful quantum computer are large numbers of fault-tolerant qubits.  Spin qubits, based on quantum dots in semiconductor materials such as silicon and germanium, are a promising platform because they resemble transistors and therefore have great potential for scalability. However, implementing error correction codes, to achieve fault tolerance, have not been shown on spin qubits yet.

Recently, we have shown a two-by-two spin qubit array in a Ge/SiGe heterostructure on a silicon wafer[1]. As a next step, we explore the possibilities of this four qubit device. Apart from the CZ gate, we demonstrate a CS gate and a native resonance SWAP gate. We use the CZ and CS gate to construct a Toffoli-like three-qubit gate. Additionally, we show that we can project the state of a data qubit unto an ancilla qubit and back. By applying echo pulses on the ancilla qubit, we can extend the coherence of the data qubit. Moreover, we can implement a majority vote phase correction code on three qubits, projecting the result unto the data qubit. These experiments are milestones for semiconductor spin qubits, since they form the first steps towards error correction on this platform.

[1] Hendrickx, et al. A four-qubit germanium quantum processor. Nature 591, 580–585 (2021).