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Updated: Jan 16, 2026
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Generation and Coherent Control of Pulsed Quantum Frequency Combs
Published on: June 8, 2018
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Strong coupling between a single-photon and a two-photon Fock state
Shuai-Peng Wang1,2,3, Alberto Mercurio4,5, Alessandro Ridolfo6
1Beijing Key Laboratory of Fault-Tolerant Quantum Computing, Beijing Academy of Quantum Information Sciences, Beijing, China.
Nature Communications
|September 30, 2025
Summary
Researchers achieved strong coupling between single photons and two-photon states in circuit quantum electrodynamics (cQED). This breakthrough enables deterministic photon-photon interactions for quantum nonlinear optics, paving the way for advanced quantum technologies.
Area of Science:
- Quantum Optics
- Quantum Information Science
- Circuit Quantum Electrodynamics (cQED)
Background:
- Strong nonlinear coupling between single photons is crucial for quantum information processing.
- Existing methods struggle to achieve deterministic, coherent photon-photon interactions.
Purpose of the Study:
- To experimentally demonstrate strong coupling between single-photon and two-photon Fock states.
- To explore a new regime of quantum nonlinear optics with individual photons.
Main Methods:
- Utilized an ultrastrongly-coupled circuit-QED system with a detuned flux qubit as a coupler.
- Exploited ultrastrong light-matter interaction and external flux bias to break conservation laws.
Main Results:
- Observed strong one-two-photon coupling by breaking excitation number and parity conservation.
- Resolved Quantum Rabi-like avoided crossing between photon states.
- Demonstrated thresholdless second harmonic generation for sub-unity mean photon numbers.
Conclusions:
- The study establishes a new regime for quantum nonlinear optics.
- Enables deterministic and coherent interaction of individual photons without external fields.
- Paves the way for all-optical deterministic quantum logic and single-photon frequency conversion.
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