Draft:Quantum-Selected Configuration Interaction

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• Comment: In accordance with the Wikimedia Foundation's Terms of Use, I disclose that I have been paid by my employer for my contributions to this article. Yoma k (talk) 15:56, 18 August 2025 (UTC)

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In quantum computing, Quantum-Selected Configuration Interaction (QSCI) is a hybrid quantum-classical algorithm that leverages a quantum computer to assist in the diagonalization of quantum observables. While initially proposed in the context of quantum chemistry to calculate the electronic structure of molecules and solids, its core principle is expected to have broader applications in various scientific and engineering domains.

From the perspective of developing methods in quantum chemistry, QSCI can be understood as a technique that uses the sampling results from a quantum computer to inform the selection of configurations for Configuration Interaction calculation^[1].

It was originally proposed in 2023 by a Japanese quantum computing company QunaSys^[1], which was followed by wider research^[2]. For example, IBM later performed a large-scale practical experiment based on this approach^[3]. They refer to the method as Sample-based quantum diagonalization (SQD). The experiment served as a notable demonstration of this hybrid approach, marking the largest-scale chemical computation on a quantum computer as of 2024.

Methodology

The QSCI involves a two-step process^[1]:

1. Quantum Selection: A quantum computer is used to prepare and measure a reference state. By analyzing the measurement outcomes, the quantum computer can efficiently identify the most dominant or "important" basis states (e.g., electronic configurations or other relevant subspaces) that are essential for accurately describing the system's ground state. The number of states to be selected is determined by a classical algorithm.
2. Classical Diagonalization: The selected basis states are then used as input for a subsequent diagonalization performed on a classical computer. Because the quantum selection process has already identified the most physically relevant subspace, the size of the classical calculation is reduced.

References

1. Kanno, Keita; Kohda, Masaya; Imai, Ryosuke; Koh, Sho; Mitarai, Kosuke; Mizukami, Wataru; Nakagawa, Yuya O. (2023). "Quantum-Selected Configuration Interaction: classical diagonalization of Hamiltonians in subspaces selected by quantum computers". arXiv. abs/2302.11320. arXiv:2302.11320.
2. Nakagawa, Yuya O.; Kamoshita, Masahiko; Mizukami, Wataru; Sudo, Shotaro; Ohnishi, Yu-ya (2024). "ADAPT-QSCI: Adaptive Construction of an Input State for Quantum-Selected Configuration Interaction". Journal of Chemical Theory and Computation. 20 (24): 10817–10825. doi:10.1021/acs.jctc.4c00846.
3. Robledo-Moreno, Javier; Motta, Mario; Haas, Holger; Javadi-Abhari, Ali; Jurcevic, Petar; Kirby, William; Martiel, Simon; Sharma, Kunal; Sharma, Sandeep; Shirakawa, Tomonori; Sitdikov, Iskandar; Sun, Rong-Yang; Sung, Kevin J.; Takita, Maika; Tran, Minh C.; Yunoki, Seiji; Mezzacapo, Antonio (2025). "Chemistry beyond the scale of exact diagonalization on a quantum-centric supercomputer". Science Advances. 11 (25): eadu9991. doi:10.1126/sciadv.adu9991.