The Institute of Physics of the Chinese Academy of Sciences has made a series of experimental progress using superconducting qubits
In recent years, superconducting quantum computing has developed rapidly, and the applications that people pay attention to are generally in two directions, the realization of quantum algorithms and the properties of quantum analog many-body systems. The use of superconducting qubits to achieve multi-particle entanglement can demonstrate the ability of the system to control multiple qubits at the same time. As a useful resource for quantum computing, quantum entanglement can be easily prepared and will reduce the difficulty of implementing quantum algorithms. However, for the use of quantum entanglement There are not many explorations to break through the standard quantum limit of the measurement accuracy of classical methods and to further approach the Heisenberg limit. This direction is the content of quantum metrology.
Quantum metrology has broad application prospects. Its purpose is to use entangled states to break through the precision limit of classical technology, in order to achieve ultra-high-precision measurement of certain physical quantities. Life experience tells us that it is difficult to measure the thickness of a piece of paper directly with a caliper, but it is easy to get the thickness of a piece of paper by measuring the thickness of a stack of paper divided by the number of layers of paper. Quantum metrology is based on this simple idea. For example, consider measuring the phase information of optical qubits. If these photons are independent of each other, according to the central limit theorem of statistics, the accuracy of multiple measurements can only reach the shot noise limit. It is called the standard quantum limit, but if all these photons are entangled to form a special multi-particle entanglement state, the phase information will be amplified, just like stacking multiple layers of paper, and then measuring the phase information can break the standard quantum limit, And it can approach the ultimate limit of precision limited by the uncertainty principle of quantum mechanics, generally known as the Heisenberg limit. This property can be called the advantage of quantum metrology.
The degree of approximation to the Heisenberg limit is related to the degree of entanglement of the multi-particle state to be detected, but the measurement of the size of multi-particle entanglement is a complex issue and also depends on the specific application people are concerned about. The advantages of quantum metrology can be used in quantum The Fisher information metric is also directly related to the entanglement size. Unfortunately, although the entanglement and quantum metrology advantages of Gaussian squeezed states can be characterized by linear compressibility, the linear compressibility cannot determine the existence of many-body entanglement for non-Gaussian entangled states in the overcompressed region. In recent years, people have noticed that the compression coefficient can be extended from the original concept of linearity to a nonlinear compression coefficient, which can well describe the entanglement degree of non-Gaussian states and is directly related to the advantages of quantum metrology, but is limited by the multi-qubit. Due to the experimental difficulty of single shot measurement, the measurement of nonlinear compressibility has not been realized in various multi-particle entangled systems.
Multiparticle entanglement can be achieved with superconducting qubits, can one obtain special entangled states with high quantum metrology advantages?
Recently, Associate Researcher Xu Kai and Fan Zhen of the Q03 Group of the Solid State Quantum Information and Computing Laboratory of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, Researcher Zheng Dongning of the SC5 Group of the State Key Laboratory of Superconductivity, and Professor Wang Haohua of Zhejiang University , in cooperation with Dr. Zhang Yuran and Professor Nori from RIKEN, Japan, using a device with 20 superconducting qubits, the newly built superconducting quantum computing platform in the Institute of Physics successfully realized the superconducting qubit multi-particle entanglement state. preparation, and combined with the measurement advantages of the system, the measurement of nonlinear compressibility was achieved for the first time.
Experiments show that the preparation of 19-bit non-Gaussian squeezed states can achieve precision very close to the Heisenberg limit, and the quantum advantage obtained is the best among the experimental results with the same number of bits. As shown in the figure below, the achieved quantum metrology advantages can be compared with entangled systems with thousands of particles in other systems, showing the advanced nature of superconducting quantum computing technology. The related results were recently published in Phys. Rev. Lett. 128, 150501 (2022).
The asterisk indicated by the arrow is the quantum metrology advantage achieved by this work, which shows that the entangled state prepared with 19 superconducting quanta is closer to the Heisenberg limit shown by the shadow boundary than other experiments.
The 19 qubit positions in the device, their mutual coupling strength information, and the experimental operation steps when measuring linear and nonlinear compressibility, quantum Fisher information.
Linear and nonlinear compressibility of entangled states of 10 superconducting qubits, and comparison of measurements of quantum Fisher information, the distribution of qubits at different points in time.
A circuit diagram of 19 qubits measuring quantum Fisher information, and the results of its distribution function are shown.
In addition, Sun Zhenghang, a researcher and doctoral student from the Institute of Physics, Chinese Academy of Sciences, cooperated with the team of Professor Zhu Xiaobo and Professor Pan Jianwei from the University of Science and Technology of China to realize a superconducting quantum device based on a 24-bit ladder structure, realizing two different properties of one-dimensional XX and ladder XX. The quantum simulation of the model has observed quantum thermalization, information scrambling and non-ergodic ergodic dynamics of integrable systems, respectively. The Institute of Physics team is responsible for the theoretical solution. The results were recently published in Phys. Rev. Lett . 128, 160502 (2022).
The above two works were respectively supported by Beijing Branch of Songshan Lake Materials Laboratory, Center of Excellence for Topological Quantum Computing of Chinese Academy of Sciences and Pilot B Project, Beijing Institute of Quantum Information Science, and National Natural Science Foundation of China.
References:
[1] Kai Xu#, Yu-Ran Zhang#, Zheng-Hang Sun#, Hekang Li, Pengtao Song, Zhongcheng Xiang, Kaixuan Huang, Hao Li, Yun-Hao Shi, Chi-Tong Chen, Xiaohui Song, Dongning Zheng, Franco Nori*, H. Wang*, and Heng Fan*, Metrological characterization of non-Gaussian entangled states of superconducting qubits, Physical Review Letters 128, 150501 (2022).
[2] Qingling Zhu#, Zheng-Hang Sun#, Ming Gong#, Fusheng Chen, Yu-Ran Zhang, Yulin Wu, Yangsen Ye, Chen Zha, Shaowei Li, Shaojun Guo, Haoran Qian, He-Liang Huang, Jiale Yu , Hui Deng, Hao Rong, Jin Lin, Yu Xu, Lihua Sun, Cheng Guo, Na Li, Futian Liang, Cheng-Zhi Peng, Heng Fan*, Xiaobo Zhu*, Jian-Wei Pan*, Observation of thermalization and information scrambling in a superconducting quantum processor, Physical Review Letters 128, 160502 (2022).
