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Unitary-transformed projective squeezing: Applications for circuit knitting and state preparation of non-Gaussian states

Keitaro Anai1,2, Yasunari Suzuki2,*, Yuuki Tokunaga2, Yuichiro Matsuzaki3, Shuntaro Takeda1, and Suguru Endo2,†

  • 1Department of Applied Physics, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
  • 2NTT Computer and Data Science Laboratories, NTT Inc., Musashino, Tokyo 180-8585, Japan
  • 3Department of Electrical, Electronic, and Communication Engineering, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan

  • *Contact author: yasunari.suzuki@ntt.com
  • †Contact author: suguru.endou@ntt.com
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Phys. Rev. Research 7, 043343 – Published 29 December, 2025

DOI: https://doi.org/10.1103/q15h-nmq9

Abstract

Continuous-variable (CV) quantum computing is a promising candidate for quantum computation because it can, even with one mode, utilize infinite-dimensional Hilbert spaces and can efficiently handle continuous values. Although photonic platforms have been considered as a leading platform for CV computation, hybrid systems that use both qubits and bosonic modes, e.g., superconducting hardware, have shown significant advances because they can prepare non-Gaussian states by utilizing the nonlinear interaction between the qubits and the bosonic modes. However, the size of hybrid hardware is currently restricted. Moreover, the fidelity of the non-Gaussian state is also restricted. This work extends the projective squeezing method to establish a formalism for projecting quantum states onto the states that are unitary-transformed from the squeezed vacuum at the expense of the sampling cost. Based on this formalism, we propose methods for simulating larger quantum devices and projecting states onto the cubic phase state, a typical non-Gaussian state, with a higher squeezing level and higher nonlinearity. To make implementation practical, we can, by leveraging the interactions in hybrid systems of qubits and bosonic modes, apply the smeared projector by using either the linear-combination-of-unitaries or virtual quantum error detection algorithms. We numerically verify the performance of our methods and show that projection can suppress the effect of photon-loss errors.

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Phys. Rev. Research 7, 043343– Published 29 December, 2025
  • Received 20 December 2024
  • Accepted 12 November 2025
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