Breakthrough in Long-Lived Quantum Entanglement

Abstract

Cooperative effects such as super(or sub)radiance in quantum systems arise from the interplay among quantum emitters. While bright superradiant states have been extensively studied and have yielded important insights into cooperative phenomena, subradiant states remain less explored due to their inherently dark state nature. However, subradiance holds significant potential as a valuable quantum resource exploiting long-lived and large-scale entanglement, which is a key for advancing quantum information technologies. Here, we demonstrate strong collective emission from a cavity-mediated steady-state subradiant state. In a tailored photonic environment with balanced cavity dissipation, emitter-field coupling strength, and incoherent pumping, two quantum dots coupled to a low-Q cavity exhibit a steady-state population in a subradiant state with a highly negative cooperativity parameter among the emitters. As a key signature of steady-state subradiance, the system shows strong photon bunching (g(2) (0) > 8) and suppressed single-photon decay (36 ns). Furthermore, we investigate that such collective interactions can be manipulated by controlling various system parameters, such as detuning and dephasing, supported by numerical simulations. Our approach to inducing cavity-mediated subradiance paves the way for generating and harnessing quantum correlations among quantum emitters via controlled dissipation.

A research team, affiliated with UNIST has successfully demonstrated the experimental creation of collective quantum entanglement rooted in dark states-previously confined to theoretical models. Unlike bright states, dark states are highly resistant to external disturbances and exhibit remarkably extended lifetimes, making them promising candidates for next-generation quantum technologies such as quantum memory and ultra-sensitive sensors.

Led by Professor Je-Hyung Kim in the Department of Physics at UNIST, in collaboration 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), the team announced that they have achieved the controlled induction of dark state-based collective entanglement. Remarkably, this entanglement exhibits a lifetime approximately 600 times longer than that of conventional bright states.

Quantum entanglement among multiple indistinguishable particles typically manifests as either bright or dark states. Dark states are characterized by their near-total invisibility to emitted light, allowing the entanglement to persist over extended periods. While this protective feature holds immense potential for quantum information storage and transmission, creating and maintaining dark states has long posed substantial experimental challenges.

The researchers overcame these obstacles by employing a nanocavity with carefully tuned loss rates, balancing the coupling strength between quantum dots and the cavity's dissipation. Dr. KyuYoung Kim, the first author of the study, explained, "When the cavity loss is too high, the quantum dots act independently without affecting each other. Conversely, if the coupling is sufficiently strong, a collective entangled state is formed, resistant to external disturbances."

In their experiments, the team observed that the entanglement within the dark state could last up to 36 nanoseconds-a lifetime approximately 600 times longer than the 62 picoseconds typical of bright states. Additionally, they detected phenomena such as non-classical photon bunching, providing direct evidence of the dark state's formation. Although such states usually suppress photon emission, under specific conditions, the entangled quantum dots emitted photons simultaneously, demonstrating the unique properties of the dark state.

Professor Kim remarked, "This experimental realization of dark state entanglement-once only theoretical-shows that by carefully engineering losses, we can preserve quantum correlations over long durations," adding "This opens new avenues for quantum information storage, high-precision sensors, and energy harvesting technologies based on quantum principles."

The findings were published online in Nature Communications on July 9, 2025. The research was supported by the National Research Foundation of Korea (NRF), the Ministry of Science and ICT (MSIT), and the Institute for Information & Communications Technology Planning & Evaluation (IITP).

Journal Reference

Kyu-Young Kim, Jin Hee Lee, Woong Bae Jeon, et al., "Cavity-mediated collective emission from steady-state subradiance," Nat. Commun., (2025).