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Strong coupling between a single-photon and a two-photon Fock state

Author

Listed:
  • Shuai-Peng Wang

    (Beijing Academy of Quantum Information Sciences
    Beijing Computational Science Research Center
    Zhejiang University)

  • Alberto Mercurio

    (École Polytechnique Fédérale de Lausanne (EPFL)
    École Polytechnique Fédérale de Lausanne (EPFL))

  • Alessandro Ridolfo

    (Università di Catania)

  • Yuqing Wang

    (Beijing Academy of Quantum Information Sciences)

  • Mo Chen

    (Beijing Academy of Quantum Information Sciences)

  • Wenyan Wang

    (Beijing Academy of Quantum Information Sciences)

  • Yulong Liu

    (Beijing Academy of Quantum Information Sciences)

  • Huanying Sun

    (Beijing Academy of Quantum Information Sciences)

  • Tiefu Li

    (Tsinghua University)

  • Franco Nori

    (RIKEN
    The University of Michigan)

  • Salvatore Savasta

    (Università di Messina)

  • J. Q. You

    (Zhejiang University)

Abstract

The realization of strong nonlinear coupling between single photons has been a long-standing goal in quantum optics and quantum information science, promising wide impact applications, such as all-optical deterministic quantum logic and single-photon frequency conversion. Here, we report an experimental observation of the strong coupling between a single-photon and a two-photon Fock state in an ultrastrongly-coupled circuit-QED system. This strong nonlinear interaction is realized by introducing a detuned flux qubit working as an effective coupler between two modes of a superconducting coplanar waveguide resonator. The ultrastrong light–matter interaction breaks the excitation number conservation, and an external flux bias breaks the parity conservation. The combined effect of the two enables the strong one–two-photon coupling. Quantum Rabi-like avoided crossing is resolved when tuning the two-photon resonance frequency of the first mode across the single-photon resonance frequency of the second mode. Within this new photonic regime, we observe the thresholdless second harmonic generation for a mean photon number below one. Our results represent a key step towards a new regime of quantum nonlinear optics, where individual photons can deterministically and coherently interact with each other in the absence of any stimulating fields.

Suggested Citation

  • Shuai-Peng Wang & Alberto Mercurio & Alessandro Ridolfo & Yuqing Wang & Mo Chen & Wenyan Wang & Yulong Liu & Huanying Sun & Tiefu Li & Franco Nori & Salvatore Savasta & J. Q. You, 2025. "Strong coupling between a single-photon and a two-photon Fock state," Nature Communications, Nature, vol. 16(1), pages 1-8, December.
    • Handle: RePEc:nat:natcom:v:16:y:2025:i:1:d:10.1038_s41467-025-63783-7
    DOI: 10.1038/s41467-025-63783-7
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    References listed on IDEAS

    as
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