May 21 2025 -- NTT Corporation has proposed a novel photonic quantum operation using high-dimensional "quantum dits (qudits)," an extension of conventional quantum bits (qubits), and demonstrated that its success rate significantly exceeds the previous theoretical limit.

Qubits realized with photons are crucial to the development of quantum information technologies—such as quantum computing and quantum communication—due to their ability to travel at the speed of light. However, a major challenge has persisted: a key operation known as the "fusion gate," essential for such technologies, has a maximum theoretical success rate of only 50%.

In this study, instead of using traditional qubits that can exist in a superposition of two states (0 and 1), the researchers theoretically showed that using qudits—quantum systems capable of existing in superpositions of three or more states—enables the implementation of fusion gates with much higher success rates, surpassing the established theoretical limit.

As a practical application, the researchers proposed a new long-distance quantum communication protocol and numerically demonstrated that it can achieve nearly an order of magnitude higher capacity compared to conventional protocols.

Importantly, the proposed photonic quantum operations can be realized using experimental techniques comparable to currently available ones, suggesting a promising path toward earlier realization of high-speed, long-distance quantum communication.

These results were published in Physical Review Letters on May 20, 2025.