Encoding and processing quantum information in the time-of-arrival of photons offer significant advantages for quantum information science and technology. These advantages include ease of experimental realization, robustness over photon state transmission, and compatibility with existing telecommunication infrastructure. Additionally, time-of-arrival encoding has the potential for high-rate quantum communication and holds promise for the future development of quantum internet. This review explores the generation, processing, and applications of time-encoded quantum states, focusing on both single-photon states, energy–time entanglement, and time-bin entanglement. We summarize the nonlinear optics platforms and advanced laser and modulation techniques utilized for photon sources that enable quantum information encoding onto the photons' time-of-arrival. We also highlight advanced quantum state processing methods in the time domain, including the Franson interferometry, optical switch-based schemes, and state-of-the-art measurement and detection schemes that allow for high-speed and multi-dimensional quantum operations. Finally, we review the mainstream implementations mainly including the quantum communication demonstrations and outline future directions for developing practical quantum networks leveraging time-encoded photon states.
Quantum optics, the study of light and its interactions with matter at the quantum level, has revolutionized modern telecommunications. By leveraging quantum phenomena such as entanglement, superposition, and photon manipulation, quantum optics enables ultra-secure communication, high-speed data transfer, and novel computing paradigms. The integration of quantum technologies into emerging communication infrastructures, particularly 6G networks, is becoming increasingly critical for overcoming the inherent limitations of classical communication and computation. In summary, quantum optics and quantum information have profoundly influenced modern physics and technology, where the field continues to bridge fundamental research and practical applications, promising further innovations in the future. The exponential growth of data traffic necessitates advancements in secure and high-speed communication technologies. Classical telecommunications rely on electromagnetic waves, but quantum optics introduces fundamentally new capabilities by exploiting quantum states of light. This paper explores the fundamental principles of quantum optics, its applications in quantum key distribution (QKD), quantum teleportation, and quantum repeaters, and its impact on next-generation telecommunications. We also discussed challenges and future prospects of the quantum optics in telecommunications.