New Study Unveils Unique Quantum State for Error-Free Quantum Computers Leveraging Light

Abstract
Two-dimensional honeycomb antiferromagnets are promising materials class for realizing Kitaev quantum spin liquids. The signature of these materials includes anisotropic bond-dependent magnetic responses and persistent fluctuations in paramagnetic regime. Here, we propose Cu3Co2SbO6 heterostructures as an intriguing candidate, wherein bond-dependent and frustrated spins interact with optical excitons. First-principles spin Hamiltonian calculations and in-plane anisotropic critical fields suggest strong frustration and dominant Kitaev exchange interactions. Optical spectroscopy reveals exciton coupled to frustrated magnetism, enabling optical detection of spin states. Spin-exciton coupling displays anisotropic responses to light polarization along the bond-parallel and the bond-perpendicular directions, highlighting Kitaev interactions and persistent short-range spin correlations above twice the Néel temperatures. The robustness of short-range spin fluctuations under magnetic fields underscores the stability of the spin-fluctuation region. Our results establish Cu3Co2SbO6 as an attractive candidate for exploring quantum spin liquid, where the spin Hamiltonian and quasiparticle excitations can be probed and potentially controlled by light.

A team of researchers, affiliated with UNIST has succeeded in identifing traces of the Kitaev Quantum Spin Liquid (QSL) using light. The Kitaev QSL represents a special quantum state that could pave the way for the development of error-free, large-scale quantum computers. However, experimentally confirmed instances of this phenomenon within materials have been scarce, prompting ongoing efforts to discover suitable candidate materials. The new experimental methodology utilizing light for detecting the characteristics of Kitaev QSL is expected to aid in the exploration and characterization of quantum computing materials.

The research team, led by Professor Changhee Sohn from the Department of Physics at UNIST, in collaboration with Professor Jae Hoon Kim’s team from Yonsei University and Professor Jung-Woo Yoo’s team from the Department of Materials Science and Engineering at UNIST, reported that they successfully detected the spin fluctuations indicative of the Kitaev QSL state in thin film cobalt-based oxides.

Kitaev QSL is a unique form of the quantum spin liquid state. In this state, spin particles within a solid do not align even at low temperatures, maintaining a fluid and disordered state akin to liquid molecules that exhibit dynamic fluctuations.

The collaborative team detected these spin fluctuations in cobalt-based oxides synthesized in a thin film format, measuring just 20 nm in thickness. While existing neutron-based analytical methods have made it easy to observe spin fluctuations in bulk materials, signals become weak and challenging to observe when materials are reduced to thin film formats, which is essential for quantum computing applications.

Figure 1. Schematics of spin exchange interaction, spin-exciton coupling and the phase diagram of Cu3Co2SbO6.

The researchers utilized an innovative approach, analyzing exciton particles generated by illuminating the thin film with light, to detect these spin fluctuations. Notably, the measured spin fluctuations persisted above a specific temperature known as the Néel temperature (16K, -257.15°C), providing evidence that these fluctuations arise not merely due to thermal effects but are a characteristic of the quantum spin liquid state. Additionally, theoretical calculations confirmed strong Kitaev interactions, typically found in Kitaev QSLs rather than standard quantum spin liquids.

Professor Sohn stated, “This research demonstrates that the characteristics of Kitaev QSL can manifest in cobalt-based oxides in thin film form. Moreover, our analytical method used in the experiment is expected to contribute significantly to the development of quantum computing materials.”

Co-first authors of the study include Baekjune Kang, Uksam Choi, and Seunghyeon Noh from UNIST, along with Taek Sun Jung from Yonsei University. The results of this research were published online in Nature Communications on February 3, 2025. The study was supported by the Ministry of Science and ICT (MSIT) and the National Research Foundation (NRF) of Korea.

Journal Reference
Baekjune Kang, Uksam Choi, Taek Sun Jung, “Optical detection of bond-dependent and frustrated spin in the two-dimensional cobalt-based honeycomb antiferromagnet Cu3Co2SbO6,” Nat. Commun., (2025).