Co-transmission of classical data and continuous-variable entanglement over a single quantum channel

Traditional protocols for simultaneous quantum-classical communication (SQCC) often rely on direct measurement of the signal, which collapses the quantum state and renders them incompatible with entanglement distribution. This paper addresses whether it is possible to integrate classical data transmission with continuous-variable (CV) entanglement distribution without destroying the quantum correlations required for quantum networking.

Building on a previously proposed classically-modulated quantum communication (CMQC) framework, the authors utilize a continuous-variable teleportation scheme at the receiver's end. Alice encodes classical information by displacing her quantum state in phase space before transmission. Upon receiving the signal, Bob performs a dual homodyne measurement on a beamsplitter, mixing the incoming signal with his own auxiliary entangled state. This measurement serves two purposes: it allows Bob to estimate Alice's classical symbol and provides the necessary information to perform a feedforward displacement operation, effectively teleporting the entanglement while simultaneously extracting the classical data.

The study demonstrates that the CMQC protocol successfully distributes Gaussian entanglement while maintaining a usable classical data stream. The authors characterize the quality of the resulting entangled state, showing that the non-Gaussian noise introduced by classical bit-error events decreases exponentially as the classical signal strength increases. They further provide a lower bound on the quantum performance of the protocol, quantifying the secret key generation rate and entanglement of formation, and show that the scheme is compatible with repeater-based quantum networks.

This protocol offers a practical path toward retrofitting existing classical communication infrastructure for quantum networking. By integrating classical and quantum information into a single hybrid signal, the scheme allows for the synchronous distribution of entanglement without requiring separate channels or significant hardware upgrades. This could enable nodes within a classical network to passively generate and store high-quality entanglement, facilitating advanced quantum tasks like teleportation and distributed quantum computing.