  In the upcoming 6G era, existing terrestrial networks have evolved toward
space-air-ground integrated networks (SAGIN), providing ultra-high data rates,
seamless network coverage, and ubiquitous intelligence for communications of
applications and services. However, conventional communications in SAGIN still
face data confidentiality issues. Fortunately, the concept of Quantum Key
Distribution (QKD) over SAGIN is able to provide information-theoretic security
for secure communications in SAGIN with quantum cryptography. Therefore, in
this paper, we propose the quantum-secured SAGIN which is feasible to achieve
proven secure communications using quantum mechanics to protect data channels
between space, air, and ground nodes. Moreover, we propose a universal QKD
service provisioning framework to minimize the cost of QKD services under the
uncertainty and dynamics of communications in quantum-secured SAGIN. In this
framework, fiber-based QKD services are deployed in passive optical networks
with the advantages of low loss and high stability. Moreover, the widely
covered and flexible satellite- and UAV-based QKD services are provisioned as a
supplement during the real-time data transmission phase. Finally, to examine
the effectiveness of the proposed concept and framework, a case study of
quantum-secured SAGIN in the Metaverse is conducted where uncertain and dynamic
factors of the secure communications in Metaverse applications are effectively
resolved in the proposed framework.
