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

Shuai-Peng Wang, Alberto Mercurio, Alessandro Ridolfo, Yuqing Wang, Mo Chen, Wenyan Wang, Yulong Liu, Huanying Sun, Tiefu Li (), Franco Nori, Salvatore Savasta () and J. Q. You ()
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Shuai-Peng Wang: Beijing Academy of Quantum Information Sciences
Alberto Mercurio: 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
Salvatore Savasta: Università di Messina
J. Q. You: Zhejiang University

Nature Communications, 2025, vol. 16, issue 1, 1-8

Abstract: 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.

Date: 2025
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DOI: 10.1038/s41467-025-63783-7

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