Breakthrough: Spontaneous Emission Reimagined in Time-Varying Media
A KAIST-led international collaboration has achieved a landmark result in quantum optics by demonstrating that spontaneous emission behaves in a fundamentally new way inside Photonic Time Crystals (PTCs). Challenging prior predictions, the team shows that the spontaneous emission decay rate is enhanced—not extinguished—by rapid, periodic changes of a medium’s refractive index in time. The work also forecasts a novel process, spontaneous emission excitation, in which an atom transitions from its ground state to an excited state while emitting a photon.
What are Photonic Time Crystals?
Photonic Time Crystals are media whose optical properties vary periodically over time rather than space. In this paradigm, the environment surrounding an emitter is not fixed; it is dynamically driven, offering a new axis—time—along which light–matter interactions can be engineered. This time-domain control complements traditional spatial methods such as resonators and photonic crystals, expanding the toolbox for shaping emission, absorption, and energy transfer at the quantum level.
Key Findings: Decay Enhancement and a New Emission Channel
Enhanced spontaneous emission decay
Contrary to a notable 2022 Science paper that suggested decay suppression under certain time-varying conditions, the KAIST team demonstrated a pronounced enhancement of the spontaneous emission decay rate when a quantum emitter is placed in a time-modulated environment. The explanation centers on non-orthogonal mode effects arising in non-Hermitian optics, where the usual assumptions about energy conservation and mode independence do not apply. This finding redefines how light couples to matter in rapidly changing media and underscores the importance of non-Hermitian considerations in time-dependent systems.
Spontaneous emission excitation
The study also predicts and reports a non-equilibrium phenomenon dubbed spontaneous emission excitation. In this process, an atom absorbs energy from the time-crystal medium and makes a transition from the ground state to an excited state while simultaneously emitting a photon. This dual action—emission with excitation—arises because the medium provides external energy through its periodic time variation, a scenario that falls outside conventional equilibrium optics and opens a new pathway for light–matter interactions in driven quantum systems.
Why This Matters: Shifting the Theoretical Landscape
These results mark a paradigm shift in spontaneous emission research. By showing that a time-periodic refractive index can amplify decay through non-orthogonal modes, the work highlights non-Hermitian physics as an essential framework for understanding time-varying photonic environments. The spontaneous emission excitation further expands the catalog of light–matter processes accessible in driven media, suggesting new mechanisms for energy exchange that could influence future quantum light sources and non-equilibrium quantum technologies.
Collaborations, Publication, and Impact
The KAIST Department of Physics team, led by Professor Bumki Min, collaborated with Professor Jonghwa Shin (Materials Science and Engineering), Professor Wonju Jeon (Mechanical Engineering), Professor Gil Young Cho (Physics), and researchers from IBS, UC Berkeley, and the Hong Kong University of Science and Technology. The research was published online in Physical Review Letters on September 23, 2025 and was featured as an Editors’ Suggestion on Physics.org. The project received support from the National Research Foundation of Korea and the Samsung Science and Technology Foundation.
Looking Ahead: Applications in Quantum Technologies
Beyond rewriting theory, these insights carry practical implications for quantum light sources, detectors, and non-equilibrium quantum optics. Designers of single-photon emitters and quantum networks may leverage time-varying media to tailor emission rates and achieve novel functionalities that are unattainable in static environments. As experimental techniques advance, the community anticipates further demonstrations of spontaneous emission excitation and other time-domain phenomena that could become standard tools in quantum engineering.
Voices from the Research Team
Professor Bumki Min remarked that the work “re-establishes the fundamental theory describing spontaneous emission in a rapidly time-varying environment.” Ph.D. candidate Kyungmin Lee, the paper’s first author, notes that the findings open new avenues for exploring light–matter interactions in non-equilibrium settings. The study stands as a landmark for time-domain photonics and a catalyst for future explorations in non-Hermitian optical systems.
