image: Fig 1: (a) Concept of cluster quantum microcombs. (b) Quadrature noise variances of EPR pairs relative to vacuum shot noise. (c) Covariance matrix of a 1D cluster state. (d) Covariance matrix of a 2D cluster state.
Credit: Wang Ze, Wang Yue et al.
A team of researchers from Peking University and the Chinese Academy of Sciences has made a breakthrough in quantum photonics by demonstrating a method to generate large-scale entangled states—known as cluster states—directly on a chip using optical microresonators. Their work, detailed in Light: Science & Applications, achieves a 60-mode cluster state, an order of magnitude larger than previous on-chip demonstrations.
In quantum technologies, cluster states are essential because they allow many quantum entities to interact in a highly coordinated way, which is critical for advanced computing, secure communications, and sensitive measurements. Traditionally, such entangled states were produced using probabilistic methods that limited their scalability. In contrast, the researchers employed a continuous-variable approach that deterministically creates entanglement, ensuring consistent and reliable operation.
The breakthrough centers on an optical microresonator—a tiny device that confines light in a circular path and features closely spaced frequency modes with high nonlinear efficiency. The team implemented a sophisticated multiple-pump technique, using up to three synchronized lasers. One primary pump initiates degenerate four-wave mixing to generate symmetric pairs of entangled light modes, while the additional pumps contribute via non-degenerate four-wave mixing to expand the entangled network further. This combination results in a vast, interconnected network of 60 entangled modes arranged in both linear and grid-like patterns.
Advanced measurement techniques, including phase-locked balanced homodyne detection, were used to directly assess the orthogonal quadratures of the light modes. By constructing a covariance matrix and applying the positive partial transpose (PPT) criterion, the researchers confirmed the stability of the entanglement links, with a measured squeezing of up to 3 dB—a world-leading squeezing level clearly indicating high-quality entanglement.
This achievement not only provides a robust experimental platform for exploring quantum entanglement but also paves the way for the development of scalable, chip-based quantum light sources. They could be the foundation for next-generation quantum computers, ultra-secure communications, and advanced sensors, all within compact and efficient devices.
Journal
Light Science & Applications
Article Title
Large-scale cluster quantum microcombs