  The concept of digital twin (DT), which enables the creation of a
programmable, digital representation of physical systems, is expected to
revolutionize future industries and will lie at the heart of the vision of a
future smart society, namely, Society 5.0, in which high integration between
cyber (digital) and physical spaces is exploited to bring economic and societal
advancements. However, the success of such a DT-driven Society 5.0 requires a
synergistic convergence of artificial intelligence and networking technologies
into an integrated, programmable system that can coordinate DT networks to
effectively deliver diverse Society 5.0 services. Prior works remain restricted
to either qualitative study, simple analysis or software implementations of a
single DT, and thus, they cannot provide the highly synergistic integration of
digital and physical spaces as required by Society 5.0. In contrast, this paper
envisions a novel concept of an Internet of Federated Digital Twins (IoFDT)
that holistically integrates heterogeneous and physically separated DTs
representing different Society 5.0 services within a single framework and
system. For this concept of IoFDT, we first introduce a hierarchical
architecture that integrates federated DTs through horizontal and vertical
interactions, bridging cyber and physical spaces to unlock new possibilities.
Then, we discuss challenges of realizing IoFDT, highlighting the intricacies
across communication, computing, and AI-native networks while also underscoring
potential innovative solutions. Subsequently, we elaborate on the importance of
the implementation of a unified IoFDT platform that integrates all technical
components and orchestrates their interactions, emphasizing the necessity of
practical experimental platforms with a focus on real-world applications in
areas like smart mobility.
