An experiment is performed to reconstruct an unknown photonic quantum state with a limited amount of copies. A semi-quantum reinforcement learning approach is employed to adapt one qubit state, an "agent," to an unknown quantum state, an "environment," by successive single-shot measurements and feedback, in order to achieve maximum overlap. The experimental learning device herein, composed of a quantum photonics setup, can adjust the corresponding parameters to rotate the agent system based on the measurement outcomes "0" or "1" in the environment (i.e., reward/punishment signals). The results show that, when assisted by such a quantum machine learning technique, fidelities of the deterministic single-photon agent states can achieve over 88% under a proper reward/punishment ratio within 50 iterations. This protocol offers a tool for reconstructing an unknown quantum state when only limited copies are provided, and can also be extended to higher dimensions, multipartite, and mixed quantum state scenarios.
@article{arxiv.1808.09241,
title = {Reconstruction of a Photonic Qubit State with Reinforcement Learning},
author = {Shang Yu and F. Albarran-Arriagada and J. C. Retamal and Yi-Tao Wang and Wei Liu and Zhi-Jin Ke and Yu Meng and Zhi-Peng Li and Jian-Shun Tang and E. Solano and L. Lamata and Chuan-Feng Li and Guang-Can Guo},
journal= {arXiv preprint arXiv:1808.09241},
year = {2019}
}