KAIST: "Quantum Operation Verification and Optimization... Opening the Door to Technological Scalability"

by Jeong Ilwoong

Published 17 Nov.2025 08:18(KST)

A technology that allows for the clear visualization and analysis of complex multi-optical mode quantum operations-much like a “CT scan”-has been developed in South Korea. This breakthrough is expected to contribute to the advancement of next-generation quantum computing and quantum communication technologies.

On November 17, KAIST announced that the research team led by Professor Youngsik Ra in the Department of Physics has developed a “Quantum Process Tomography” technology, which enables the rapid and accurate identification of the characteristics of multi-optical mode quantum operations occurring inside quantum computers.

(From left) Gunkhee Kwak, Integrated MS-PhD Program in Physics at KAIST; Youngsik Ra, Professor; Chan Noh, Postdoctoral Researcher; Youngdo Yoon, Integrated MS-PhD Program (Top left) Myungsik Kim, Professor at Imperial College London. Provided by KAIST

Tomography is a technology that reconstructs invisible internal structures using diverse data, much like medical CT scans. In quantum computing, it is essential to have a technique that reconstructs the internal operating principles of quantum operations based on various experimental data.

For quantum computers to outperform classical computers, they must be able to manipulate a large number of quantum units (qubits or modes) simultaneously.

However, as the number of qubits or optical modes increases, the workload required for tomography grows exponentially, making it difficult for existing technologies to analyze even five optical modes.

In contrast, the technology developed by the research team can clearly map out the processes occurring within quantum operations, similar to CT imaging.

To achieve this, the team introduced a new mathematical representation that precisely describes nonlinear optical processes.

Inside a quantum computer, multiple light signals interact and become intricately entangled in highly complex ways. The mathematical representation analyzes these complex quantum states by using an “amplification matrix” to show how the light is amplified and transformed, and a “noise matrix” to indicate how much noise or loss-caused by the external environment-has been mixed in.

This approach enables the creation of a “quantum state map” that can accurately identify changes in the inherent quantum properties of light (ideal changes) and unavoidable noise (non-ideal changes) either separately or simultaneously, thereby allowing a more realistic understanding of quantum computer operations.

In particular, the research team precisely observed how various types of “light signals (quantum states)” changed after being input. They then used a statistical method called maximum likelihood estimation to reverse-track and determine “what operations actually occurred inside.”

As a result, they became the first in the world to experimentally clarify a large-scale quantum operation involving 16 optical modes (light signals) entangled and operating together. This achievement overcame the previous limitation of analyzing only up to five modes, as the amount of required analysis increased explosively with each additional mode. The breakthrough was made possible by significantly reducing the necessary computational workload.

Professor Ra stated, “This research is significant in that it dramatically improves the efficiency of quantum process tomography, a core foundational technology for quantum computing. The technology we have secured will contribute to enhancing the scalability and reliability of various quantum technologies, including quantum computing, quantum communication, and quantum sensing.”

Meanwhile, Gunkhee Kwak, an integrated MS-PhD student in the Department of Physics, participated as the first author, while Chan Noh, a postdoctoral researcher, Youngdo Yoon, an integrated MS-PhD student, and Professor Myungsik Kim of Imperial College London participated as co-authors.