New progress in the scaling of semiconductor quantum dot based qubit has been achieved at Key Laboratory of Quantum Information and Synergetic Innovation Center of Quantum Information & Quantum Physics of USTC in China. Professor GUO Guoping with his co-workers, XIAO Ming, LI Haiou and CAO Gang, have designed and fabricated a quantum processor with six quantum dots, and experimentally demonstrated quantum control of the Toffoli gate. This is the first time for the realization of the Toffoli gate in the semiconductor quantum dot system, which motivates further research on larger scale semiconductor quantum processor. The result was published as 'Controlled Quantum Operations of a Semiconductor Three-Qubit System ' (Physical Review Applied 9, 024015 (2018)).

Developing the scalable semiconductor quantum chip that is compatible with modern semiconductor-techniques is an important research area. In this area, the fabrication, manipulation and scaling of semiconductor quantum dot based qubits are the most important core technologies. Professor GUO Guoping's group aims to master these technologies and has been devoted to this area for a long time. Before the demonstration of the three-qubit gate, they have realized ultrafast universal control of the charge qubit based on semiconductor quantum dots in 2013 (Nature Communications. 4:1401 (2013)), and achieved the controlled rotation of two charge qubits in 2015 (Nature Communications. 6:7681 (2015)).

The Toffoli gate is a three-qubit operation that changed the state of a target qubit conditioned on the state of two control qubits. It can be used for universal reversible classical computation and also forms a universal set of qubit gates in quantum computation together with a Hadamard gate. Furthermore, it is a key element in quantum error correction schemes. Implementation of the Toffoli gate with only single- and two-qubit operations requires six controlled-NOT gates and ten single-qubit operations.

As a result, a single-step Toffoli gate can reduce the number of quantum operations dramatically, which can break the limit of coherence time and improve the efficiency of quantum supercomputing. Researchers from Guo's group found the T-shaped six quantum dot architecture with openings between control qubits and the target qubit can strengthen the coupling between qubits with different function and minimize it between qubits with the same function, which satisfies the requirements of the Toffoli gate well. Using this architecture with optimized high frequency pulses, researchers demonstrated the Toffoli gate in semiconductor quantum dot system in the world for the first time, which paves the way and lays a solid foundation for the scalable semiconductor quantum processor.

The reviewer spoke highly of this work, and thought this is an important progress in the field of semiconductor quantum dot based quantum supercomputing. "The work is detailed and clearly demonstrates a high level of experimental technique and would be of high interest to people working in the field of electrostatically defined quantum dots for quantum computation."