Breakthrough: Spontaneous Emission Gets a Time-Tuned Twist
A KAIST-led collaboration has unveiled a surprising twist in how light interacts with matter when the environment itself changes in time. In a study published online in Physical Review Letters on September 23, 2025, researchers from KAIST’s Department of Physics—led by Professor Bumki Min—and partners from the Department of Materials Science and Engineering, Mechanical Engineering, and Physics, along with researchers from IBS, UC Berkeley, and the Hong Kong University of Science and Technology, report a fundamental shift in the behavior of spontaneous emission inside a Photonic Time Crystal (PTC). The work challenges earlier predictions and introduces a new, non-equilibrium phenomenon labeled as spontaneous emission excitation.
What is a Photonic Time Crystal and Why It Matters
Photonic Time Crystals are media in which the refractive index is modulated periodically over time, rather than in space. This temporal modulation creates a unique playground for light–matter interactions, offering a way to control emission, absorption, and energy exchange along the time axis itself. The KAIST team’s analysis shows that, when a quantum emitter sits inside such a rapidly changing environment, the decay dynamics of spontaneous emission do not simply shut off at special frequencies as some theories once suggested. Instead, the decay rate is notably enhanced. This enhancement arises from the so-called non-orthogonal mode effect, underscoring the importance of non-Hermitian optics in time-varying systems.
From Decay Suppression to Decay Enhancement
Earlier theory, including a widely cited Science paper from 2022, anticipated that a Photonic Time Crystal could suppress spontaneous emission decay at a particular frequency. The new work reverses that expectation, showing that the dynamic, time-dependent medium can reinforce the emission process. The researchers attribute this to a non-orthogonal mode effect that emerges when the optical modes in the time-modulated medium are not strictly independent. In practical terms, this means an excited atom can release a photon more readily than in a static or purely spatially structured environment, opening pathways to faster, more efficient light sources and novel quantum devices that exploit temporal control rather than just spatial design.
Spontaneous Emission Excitation: A Ground-State Jump Fueled by Time
Beyond the enhanced decay, the study predicts a striking new process: spontaneous emission excitation. In this non-equilibrium scenario, an atom can transition from its ground state to an excited state while simultaneously emitting a photon. The energy for this excitation comes from the time-varying medium itself, which injects external energy into the system. This phenomenon cannot be explained by conventional equilibrium optics and highlights a richer class of light–matter interactions made possible by time-periodic modulation. If realized experimentally, spontaneous emission excitation could enable new ways to prepare and manipulate quantum states using time-domain engineering instead of, or in addition to, traditional energy-pumping schemes.
Implications for Quantum Technologies and Non-Equilibrium Optics
These findings reframe how scientists think about spontaneous emission in non-stationary environments. The demonstrated decay-rate enhancement and the proposed excitation pathway broaden the toolkit for designing quantum light sources, sensors, and information-processing elements that rely on time-domain control. The results also deepen our understanding of non-Hermitian optics, a field that studies systems where energy exchange with the environment cannot be neglected. In practical terms, Photonic Time Crystals could enable faster, more tunable quantum emitters and pave the way for devices that harness non-equilibrium dynamics to achieve new functionalities in quantum communication and computation.
About the Research and What Comes Next
Professor Bumki Min described the work as a re-establishment of the fundamental theory describing spontaneous emission in rapidly time-varying environments. The team, led by Ph.D. candidate Kyungmin Lee as first author, emphasizes that the enhanced decay and the spontaneous emission excitation hold promise for broad applications in quantum optics. The study, titled “Spontaneous emission decay and excitation in photonic time crystals,” was supported by the National Research Foundation of Korea and the Samsung Science and Technology Foundation. The findings were published online in Physical Review Letters and have drawn attention on Physics.org as an Editors’ Suggestion piece, signaling broad interest in the physics community regarding time-domain control of light–matter interactions.
About the Collaboration
The KAIST team collaborated with scholars from the KAIST Department of Materials Science and Engineering, KAIST Department of Mechanical Engineering, and KAIST Department of Physics, along with researchers from IBS, UC Berkeley, and the Hong Kong University of Science and Technology. The cross-institutional effort reflects a growing global interest in harnessing time-modulated media to explore non-equilibrium quantum physics and to redefine strategies for quantum light source design.
