RQC Seminar

202nd RQC Seminar

• Speaker

  Prof. Salvatore Savasta
  ( University of Messina, Italy )

• Date

  16:00-17:00 (4:00 p.m. - 5:00 p.m.), May 28, 2025 (Wednesday)

• Venue

  Hybrid(ZOOM・ Wako Main Research 3F 345-347 Seminar Room / 研究本館3階 セミナー室 (345-347) (C01))

• Title

  Strong Coupling between a Single-Photon and a Two-Photon State

• Inquiries

  norilab_rqc_assist[at]ml.riken.jp

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 [1]. 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, following the proposal in Ref.[2]. The ultrastrong light-matter interaction breaks the excitation number conservation, and an external flux bias breaks the parity conservation [3]. 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 thresholdless second harmonic generation for a mean photon number in the resonator below one. These results represent a significant 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 field.

[1] S.P. Wang et al., Strong Coupling between a Single-Photon and a Two-Photon Fock State, arXiv preprint arXiv:2401.02738 (2025).

[2] A.F. Kockum et al., Deterministic Quantum Nonlinear Optics with Single Atoms and Virtual Photons, Phys. Rev. A 95, 063849442 (2017).

[3] A.F. Kockum et al., Ultrastrong Coupling between Light and Matter, Nat. Rev. Phys. 1, 19 (2019).