Developing the scalable semiconductor quantum chip compatible with modern semiconductor-techniques is an important research area where the fabrication, manipulation and scaling of semiconductor quantum dot based qubits are the most important core technologies.

The Toffoli gate is a three-qubit operation that changed the state of a target qubit conditioned on the state of two control qubits. A single-step Toffoli gate can reduce the number of quantum operations dramatically, breaking the limit of coherence time and improving the efficiency of quantum computing. The ability to demonstrate gate operations beyond the two-qubit limit is, therefore, of great technical and scientific value.

Prof. GUO Guoping and the co-workers at Key Laboratory of Quantum Information and Synergetic Innovation Center of Quantum Information & Quantum Physics of University of Science and Technology of China of Chinese Academy of Sciences designed and fabricated a quantum processor with three capacitively coupled double quantum dots, and experimentally demonstrated quantum control of a three-qubit Toffoli gate. The result was published in Physical Review Applied.

Prof. GUO’s group has been devoted to the development of the scalable semiconductor quantum chip for a long time. Before the demonstration of the three-qubit gate, they have realized ultrafast universal control of the single charge qubit based on semiconductor quantum dots in 2013, and achieved the controlled rotation of two charge qubits in 2015.

In this study, the researchers found that the T-shaped quantum dot architecture with openings between control qubits and the target qubit satisfies the requirements of the Toffoli gate well.

Combing the T-shaped triple-dot array with the architecture of a linear array, they achieved strong and tunable coupling between a target qubit and two control qubits, and showed experimentally that the target qubit’s quantum rotations, including both amplitude and phase rotations, can be switched on and off nearly completely by setting the two control qubits to certain eigenstates.

The functionalities of a Toffoli gate were, therefore, demonstrated in semiconductor quantum dot system in the world for the first time, which paves the way for the scalable semiconductor quantum processor.

"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”, said the reviewer.

The work was supported by the Ministry of Science, the National Natural Science Foundation of China, Chinese Academy of Sciences and the Ministry of Education.

Figure: The Toffoli Gate in Three-Qubit System (Image by GUO's group)