Kyoto University solves major quantum imaging challenge, sets resolution record

A research group consisting of Program-Specific Researcher Naofumi Abe (at the time of the research), Master's Student Tomoyasu Horino (at the time of the research), Master's Student Yoko Kawaguchi (at the time of the research), Associate Professor Ryo Okamoto, and Professor Shigeki Takeuchi of the Graduate School of Engineering at Kyoto University successfully demonstrated Quantum Optical Coherence Tomography (QOCT) imaging with the world's highest volumetric resolution of 4.04 femtoliters. This achievement represents a 43-fold improvement over conventional QOCT systems. In addition, the team developed a new graph theory-based algorithm utilizing maximum clique search, enabling high-speed removal of artifact signals, a major challenge in QOCT imaging. These advances make it possible to noninvasively observe fine three-dimensional structures at high resolution, with potential applications across a wide range of fields, including materials science and the life sciences. The study was published online in Optics Express.

Optical Coherence Tomography (OCT) is an imaging technique that uses optical interference to detect light reflected from within a sample and visualize its internal structure. It is widely used for retinal imaging in ophthalmology and for non-destructive inspection in industrial manufacturing. However, conventional OCT is susceptible to image degradation caused by optical dispersion, which limits achievable resolution.

Research on optical quantum sensing, which surpasses the limitations of conventional measurement technologies by controlling the behavior of individual quanta and their correlations (quantum entanglement), is advancing rapidly. In particular, Quantum Optical Coherence Tomography (QOCT), which uses quantum-entangled light, has attracted considerable attention because it is inherently immune to dispersion within samples, enabling higher-resolution imaging than conventional OCT. However, technical challenges have limited the volumetric resolution of QOCT to 176 femtoliters, and the technique has also been affected by unique artifact signals that appear in images.

By refining the optical system, the research group developed a light source capable of generating broadband frequency-entangled photons. They also designed and constructed an optical system in which chromatic aberration, a phenomenon in which the focal position varies with wavelength, was effectively suppressed even for such broadband entangled light. As a result, they achieved the world's highest volumetric resolution in QOCT and demonstrated dispersion tolerance even at this level of optical resolution.

In experiments, the researchers confirmed that automatic dispersion cancellation was maintained without any loss of resolution, even when a zinc selenide (ZnSe) plate with strong optical dispersion was inserted into the optical path. In addition, they developed a new graph theory-based algorithm employing maximum clique search, enabling the rapid removal of artifact signals that had long been a major obstacle to QOCT imaging.

The team focused on the characteristic that artifact signals appear at positions midway between two reflective surfaces. Recognizing that this behavior could be formulated as a maximum clique search problem in graph theory, they developed an algorithm to identify and remove artifact signals. By applying this approach, they reduced analysis time from several minutes to approximately 0.2 seconds (237 milliseconds) on a standard laptop computer.

Takeuchi commented: "The idea of applying graph theory was actually conceived by Horino while he was still an undergraduate student. Although he was enrolled in the Department of Electrical and Electronic Engineering, he was also able to take courses in information science. Drawing on knowledge gained through those courses, he proposed the original concept, which he later developed further together with Associate Professor Okamoto."

Approximately three years ago, Takeuchi and his colleagues established the Photonic Quantum Sensing Social Implementation Consortium, which now includes more than ten participating companies. Discussions aimed at the practical implementation and commercialization of the technology are already underway within the consortium.

Journal Information
Publication: Optics Express
Title: High-volume-resolution quantum optical coherence tomography imaging with efficient artifact removal
DOI: 10.1364/OE.574265