Strong coupling between a single-photon and a two-photon Fock state
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.
Authors
- Salvatore Savasta (ORCID: https://orcid.org/0000-0002-9253-3597)
- Huanying Sun (ORCID: https://orcid.org/0000-0003-3527-459X)
- Shuai-Peng Wang (ORCID: https://orcid.org/0000-0002-2621-5160)
- Franco Nori (ORCID: https://orcid.org/0000-0003-3682-7432)
- J. Q. You (ORCID: https://orcid.org/0000-0002-5823-549X)
- A. Ridolfo (ORCID: https://orcid.org/0000-0001-9628-6602)
- Alberto Mercurio (ORCID: https://orcid.org/0000-0001-7814-1936)
- Mo Chen (ORCID: https://orcid.org/0000-0003-1841-7608)
- Yulong Liu (ORCID: https://orcid.org/0000-0002-6884-5363)
- Yuqing Wang (ORCID: https://orcid.org/0000-0001-7170-234X)
- Wenyan Wang (ORCID: https://orcid.org/0000-0002-7486-8342)
- Tiefu Li (ORCID: https://orcid.org/0009-0007-5851-6519)
Institutions
- University of Messina (IT)
- University of Michigan (US)
- University of Catania (IT)
- Zhejiang Lab (CN)
- Beijing Computational Science Research Center (CN)
- Computing Center (RU)
- Beijing Academy of Quantum Information Sciences (CN)
- École Polytechnique Fédérale de Lausanne (CH)
- Zhejiang University (CN)
- Tsinghua University (CN)
Publication Details
- Journal
- Nature Communications
- Published
- 2025-09-30
- DOI
- https://doi.org/10.1038/s41467-025-63783-7
- Citations
- 2
- Primary Topic
- Quantum Information and Cryptography
- Type
- article
- Field-Weighted Citation Impact
- 4.96
Funders
- National Natural Science Foundation of China
- Tsinghua University
- Beijing Municipal Science and Technology Commission
- QuantERA
- Japan Science and Technology Agency
- National Key Research and Development Program of China
- Core Research for Evolutional Science and Technology
- Moonshot Research and Development Program
- Army Research Office