Entangling Schrödinger’s cat states by bridging discrete- and continuous-variable encoding

Daisuke Hoshi, Toshiaki Nagase, Sangil Kwon (), Daisuke Iyama, Takahiko Kamiya, Shiori Fujii, Hiroto Mukai, Shahnawaz Ahmed, Anton Frisk Kockum, Shohei Watabe, Fumiki Yoshihara and Jaw-Shen Tsai
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Daisuke Hoshi: Tokyo University of Science
Toshiaki Nagase: Tokyo University of Science
Sangil Kwon: Tokyo University of Science
Daisuke Iyama: Tokyo University of Science
Takahiko Kamiya: Tokyo University of Science
Shiori Fujii: Tokyo University of Science
Hiroto Mukai: RIKEN Center for Quantum Computing (RQC)
Shahnawaz Ahmed: Chalmers University of Technology
Anton Frisk Kockum: Chalmers University of Technology
Shohei Watabe: Tokyo University of Science
Fumiki Yoshihara: Tokyo University of Science
Jaw-Shen Tsai: RIKEN Center for Quantum Computing (RQC)

Nature Communications, 2025, vol. 16, issue 1, 1-10

Abstract: Abstract In quantum information processing, two primary research directions have emerged: one based on discrete variables (DV) and the other on the structure of quantum states in a continuous-variable (CV) space. Integrating these two approaches could unlock new potentials, overcoming their respective limitations. Here, we show that such a DV–CV hybrid approach, applied to superconducting Kerr parametric oscillators (KPOs), enables us to entangle a pair of Schrödinger’s cat states by two methods. The first involves the entanglement-preserving conversion between Bell states in the Fock-state basis (DV encoding) and those in the cat-state basis (CV encoding). The second method implements a $$\sqrt{{{{\rm{iSWAP}}}}}$$ iSWAP gate between two cat states following the procedure for Fock-state encoding. This simple and fast gate operation completes a universal quantum gate set in a KPO system. Our work offers powerful applications of DV–CV hybridization and marks a first step toward developing a multi-qubit platform based on planar KPO systems.

Date: 2025
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DOI: 10.1038/s41467-025-56503-8

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