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Direct quantum process characterization via erasure-based closed timelike curves

Ran Yang1, Gang Yang1, Wei Zhou1, Dong-Jie Guo1, Xinhui Li1, Ji-Ning Zhang1, Yu Cui1, Yan-Xiao Gong1,2,*, and Shi-Ning Zhu1

  • 1National Laboratory of Solid State Microstructures, School of Physics, Jiangsu Key Laboratory of Quantum Information Science and Technology, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
  • 2Hefei National Laboratory, Hefei 230088, China

  • *Contact author: gongyanxiao@nju.edu.cn
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Phys. Rev. Research 8, 043075 – Published 19 October, 2026

DOI: https://doi.org/10.1103/bskn-3gy3

Abstract

Characterizing complex quantum evolutions in the microscopic world is crucial for understanding fundamental physics and advancing quantum information applications. In conventional quantum process tomography, individual process matrix elements are obtained indirectly through reconstruction. Here, we use sequential system-ancilla couplings and quantum erasure to directly access these elements for arbitrary quantum processes. Within the pseudo-density-matrix framework, these operations realize an effective erasure-based closed timelike curve (E-CTC) interaction. During E-CTC-assisted measurements, the protocol combines selective computational-basis addressing with fixed superposition preparation and projections. Although the configuration count is not reduced for general processes, these fixed settings can simplify control when computational-basis states are readily addressable. We experimentally demonstrate our scheme in a photonic platform by characterizing several two- and four-dimensional quantum processes with average fidelities above 99%. Our work offers a basis-rotation-free approach to high-dimensional quantum process characterization within the erasure-based CTC framework.

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Phys. Rev. Research 8, 043075– Published 19 October, 2026
  • Received 14 November 2025
  • Accepted 2 October 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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