A promising development for practical quantum technologies has been leveraged by researchers at South Korea’s Ulsan National Institute of Science and Technology (UNIST), who finally achieved dark state quantum entanglement, a state previously only existing in theoretical models.
One of the largest stumbling blocks to commercial quantum technologies is their lack of resilience, which requires strict controlled conditions. Even then, they are prone to errors. However, these dark states are much more resistant to external disturbances than “bright” states, allowing for extended lifetimes that may power practical quantum memory and sensor applications in the future.
Empowering Quantum Technology
The team behind the new research was led by UNIST’s Professor Je-Hyung Kim, working with Dr. Changhyoup Lee from the Korea Research Institute of Standards and Science (KRISS) and Dr. Jin Dong Song from the Korea Institute of Science and Technology (KIST).
Their work demonstrated a dark entanglement capable of outlasting bright states by 600 times, a significant boost to quantum resilience. This marks the first time such a state has been practically achieved, after extensive study of bright state quantum entanglement, which has yielded important insights into entanglement that the team built on.
When multiple indistinguishable particles enter quantum entanglement, it is usually in what are called “bright” or “dark” states. The dark states exist at almost total invisibility to emitted light, which offers protection for the entanglement to extend its lifespan.
Developing Dark State Quantum Entanglement
While the theoretical benefits of that protection have long been apparent, producing and maintaining dark state quantum entanglement has proved elusive.
A nanocavity featuring precisely calibrated loss rates was the key to the team’s feat. By balancing the cavity’s dissipation with the coupling strength between quantum dots, the team achieved the desired state.
“When the cavity loss is too high, the quantum dots act independently without affecting each other,” said lead author Dr KyuYoung Kim. “Conversely, if the coupling is sufficiently strong, a collective entangled state is formed, resistant to external disturbances.”
Usually, bright state quantum entanglements expire within 62 picoseconds, yet the South Korean researchers measured dark states lasting up to 36 nanoseconds, a 600-fold increase in duration.
Unique Properties Measured
When analyzing their attempts to produce the entanglement, the team also detected non-classical photon bunching, a form of direct evidence that they had indeed created the dark state entanglement.
While dark states typically suppress photon emission, the state offers a unique property whereby the entangled quantum dots may emit photons simultaneously under specific conditions.
“This experimental realization of dark state entanglement—once only theoretical—shows that by carefully engineering losses, we can preserve quantum correlations over long durations,” Dr Kim said. “This opens new avenues for quantum information storage, high-precision sensors, and energy harvesting technologies based on quantum principles.”
The paper, “Cavity-mediated Collective Emission from Steady-state Subradiance,” appeared in Nature Communications on July 9, 2025.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.