“Zhuangzi” superconducting quantum chip interprets “Zhuang Zhou dream butterfly”

The prospect of quantum computing is exciting, and it has potential application value in basic scientific research, new material and drug development, brain-like artificial intelligence technology development and other fields. However, the shortcomings of superconducting quantum chips such as long decoherence time and high control accuracy are still the focus of scientists.

Recently, Fan Huan, researcher and associate researcher Xu Kai, researcher of the Laboratory of Solid-State Quantum Information and Computing of the Institute of Physics, Chinese Academy of Sciences, and Zheng Dongning, researcher of the Quantum Computing Research Center of the Institute of Physics, Chinese Academy of Sciences, and Xiang Zhongzhong, deputy chief engineer, cooperated to develop a one-dimensional superconducting quantum chip with more than 40 bits, named after the Warring States period thinker and philosopher “Zhuangzi”, and used it to successfully simulate the energy spectrum of the “Hustadter butterfly” and various novel topological zero modes. The research results were published in the journal Physical Review Letters.

The birth of the “Zhuangzi” chip

In the eyes of scientists, large-scale quantum computing is developing in the direction of practical use, and to achieve practical use, it is necessary to manipulate accurately, have a large number of bits, have a long coherence time, and have high enough efficiency. In this process, the design, fabrication, measurement and control of quantum chips are crucial.

Xiangzhong, who has long been engaged in the preparation of superconducting quantum chips, told China Science News that compared with traditional chips, quantum chips are very sensitive to disturbances in the external environment, “Quantum chips are a very fragile system, the stability time is very short, running quantum algorithms on the chip is like building a snowman in the summer, and it needs enough speed to make the snow out before it melts.” Usually the coherence time of superconducting quantum chips is about tens of microseconds, which means that the quantum effect lasts only a moment, so it is difficult for us to accurately execute quantum algorithms in a short coherent time scale. ”

With the help of the superconducting quantum computing experimental platform of the Comprehensive Extreme Conditions Laboratory of the Institute of Physics of the Chinese Academy of Sciences, Zheng Dongning and Xiang Zhongzhong, after repeated exploration and thinking in the practice of device design and preparation, continuously improved and optimized the design method and preparation process of the device, completed the design and preparation of 43-bit one-dimensional superconducting quantum chip, the consistency of the overall bit parameters and design values in the chip, the overall decoherence time, the preparation yield, the crosstalk of measurement and control signals, The elimination of parasitic modes and the legibility of quantum states have been greatly improved. Some bit decoherence times can reach the order of 100 microseconds.

Observation of the “Hofstadter butterfly” energy spectrum (provided by interviewee)

The work designed and constructed various instances of diagonal AAH models of up to 41 qubits, and applied dynamic spectroscopy to experimentally measure the energy spectrum of the famous “Hofstadter butterfly”. Due to the topological characteristics of the diagonal AAH model, there is a “wing-shaped” energy gap, and the entire energy spectrum looks like a dancing butterfly, reminiscent of the story of the Zhuang Zhou Meng butterfly in the Warring States period, which is also the origin of the name of the quantum processor.

Because the Zhuangzi processor has enough qubits, the effect of the finite size effect is greatly suppressed, and the fractal structure and band splitting in the details of the butterfly’s body are clearly displayed.

Experiment of minus 200 degrees

Quantum chips are the first step, and the use of multiple superconducting qubits to simulate various quantum effects is also a cutting-edge research that people are currently focusing on.

This chip is the size of a fingernail. After getting the chip, Xu Kai and his team members immediately began to measure and control the quantum chip and carry out quantum simulation experiments. Xu Kai told China Science News, “Quantum simulation is to build some important many-body models by regulating quantum chips, so as to realize the simulation and calculation of various novel physics and properties of real substances or material systems to solve a series of important problems in energy, materials and other fields.” ”

Superconducting quantum computing chips cannot work at room temperature, and it is necessary to avoid heat (noise) interference with quantum states in extremely low temperature environments.

The researchers packaged the chip into a box and put it in a dilution refrigerator to cool down to 10mK, the temperature of the refrigerator is only 0.01 °C higher than absolute zero (minus 273.15 °C), this extremely low temperature can make the chip into a lossless superconducting state and effectively suppress the environmental noise and thermal noise around the chip, thereby presenting quantum effects, and researchers can better manipulate quantum effects.

The process of manipulating the chip is not easy. In the lab, dozens of instruments are connected to the “chip” with microwave pulse signals, and the researchers program the instrument on a software platform they have developed, sending “instructions” to the chip, thereby “manipulating” the chip. The “instruction” is issued in nanoseconds.

“We need to optimize the regulatory parameters of each qubit and the interaction between them very finely, and this process requires two months of preparation,” Xu said, adding that by automating the parameter search and automated manipulation of the program, future research will be more efficient.

Since the Zhuangzi quantum processor has more than 40 qubits, this is enough for researchers to capture a large number of its topological features in the complex band structure of this important one-dimensional quantum many-body system. Using a superconducting quantum processor aided by a highly controllable Floquet (cycle-driven) regulatory technique, this work establishes a general hybrid quantum simulation method to explore quantum topological systems in the noisy medium-scale quantum era.

Promising prospects and talent are needed

Xu Kai and Xiang Zhongzhong and his team have long been committed to experimental research in superconducting quantum computing, quantum simulation and quantum devices, and have achieved many leading results.

But in their view, the prospects are broad and there is still a long way to go.

“Although the current quantum chip can only complete some specific tasks, and has not yet reached the quantum advantage of surpassing classical computing, various manipulation technologies can be accumulated through quantum simulation experiments, and various application scenarios of quantum computing can be explored and displayed, which is very valuable for the realization and application of quantum computers in the future.” Xu Kai said.

In the eyes of researchers, compared with the best team in the world, there is still a certain gap in quantum computing in China. In Xu Kai’s view, quantum computing is an interdisciplinary discipline that requires talents from all walks of life, and they are eager to join the quantum team. “We need to build an all-round ecology. But we still have to respect the natural laws of scientific development, on the one hand, we must speed up the pace of experiments, on the other hand, we must not be too hasty. Xu Kai said. (Source: China Science News Han Yangmei)

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