Scientists from Institute of Physics of the Chinese Academy of Sciences led by Profs. FAN Heng and ZHENG Dongning as well as Zhejiang University led by Prof. WANG Haohua, together with collaborators, successfully created Schrödinger cat states of up to 20-qubits with a superconducting quantum processor.

In addition, they showed that the generated 18-qubit Greenberger-Horne-Zeilinger (GHZ) state is genuine entangled. So far, it is the largest GHZ state ever created in solid state quantum computation platforms. The achievement of those results is a big step toward building a quantum computer.

The superconducting qubit based on Josephson junction is one of the most promising schemes to build a quantum computer, because of its long coherence time, high fidelity control and precise readout.

In particular, scalability is one of the main advantages for the superconducting quantum processor, which can integrate a great number of qubits. To coherently control all qubits in a superconducting quantum processor, it is necessary to show that each qubit can be controlled precisely, and any pair of qubits can be entangled. It may be more challenging to show that all qubits can be entangled to generate a GHZ state and Schrödinger cat states.

The generation of multiqubit entanglement is a benchmark for demonstrating the performance of quantum devices and the control. To achieve those aims, the scientists from China peculiarly designed an all-to-all device with all qubits connected with others.

Altogether, there are 20 qubits integrated into the superconducting quantum device, all have long coherence time and can be controlled and readout with high fidelity.

In experiment, the system of qubits initialized coherently evolves to multi-component atomic Schrödinger cat states, i.e., superpositions of atomic coherent states including GHZ state, at specific time intervals. The fidelity of the generated 18-qubit GHZ state is larger than the threshold to demonstrate the state is genuine multiqubit entanglement.

This study entitled "Generation of multi-component atomic Schrödinger cat states of up to 20 qubits" was published on Science.

The study was supported by the National Science Foundation, the Ministry of Science and Technology of China and the Chinese Academy of Sciences.