New scheme for controlling qubits in a multilevel system

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A schematic diagram illustrating the evolution of multilevel systems using an equivalent model. Credit: Zhou Yuan et al.

A team led by Prof. Guo Guangcan of the University of Science and Technology of China (USTC) has made significant progress in research on tuning multilevel quantum systems.

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In collaboration with Professor Hu Xuedong of the University of Buffalo, State University of New York and Origin Quantum Computing Company Limited, Professors Guo Guoping, Li Haiou and Gong Ming have proposed a new type of quantum gates that can achieve control of noise-resistant qubits by fine-tuning the driving field parameters. Their work has been published in Applied Physics Review.

Quantum state manipulation is widely applied in quantum systems such as superconducting qubits and semiconducting quantum dots. A quantum system with simple energy levels is easy to manipulate, but interference can occur in a more complicated multilevel system. For example, a two-qubit semiconductor spin system has a theoretical model of five energy levels.

When driving such a system, several consistent processes within the system interfere with each other, making it difficult to analyze and control the evolution process. Currently, related research is mostly limited to various rough conditions, which are unfavorable for the further development of qubit manipulation.

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To study the effects of driving fields on multilevel systems, previous work has often relied on numerical simulations or reduced multilevel systems to two-level systems. However, these methods cannot fully describe the complex phenomena in the experiments. Therefore, finding a suitable frame of reference (or basis vector) can greatly simplify the problem.

In this work, the researchers coupled a shuttle state with all other energy levels and obtained an equivalent coupling between any two energy levels by adjusting the amplitude and frequency of the shuttle state. This is possible because the actual model of their Floquet engineering can achieve any desired equivalent model by adjusting these parameters.

The results show that within the range of experimental parameters, this approach can implement a wide range of couplings while maintaining a high control speed. Using this method, the researchers have theoretically demonstrated single-qubit and two-qubit gate operations with fidelity greater than 99%. This model can even interpret some new, previously unexplained odd-even effects observed in experiments.

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In this scheme, the shuttle state plays a crucial role. It not only allows effective coupling between any two energy levels, but also serves as a means of measurement. Researchers can conduct non-destructive measurements of quantum states by measuring the shuttle state.

This theoretical proposition has significant applications, since the multi-level energy systems discussed in this study are found in nearly all other physical systems, including superconducting atoms, ions, and qubits.

By making appropriate improvements to the scheme and selecting suitable parameters, it is possible to realize arbitrary gate control in other models. This new scheme provided new experimental insights into quantum gate operations in multilevel systems.

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More information:
Yuan Zhou et al, Full Tunability and Quantum Coherent Dynamics of a Driven Multilevel System, Applied Physics Review (2023). DOI: 10.1103/PhysRevApplied.19.044053

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Provided by the University of Science and Technology of China

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