Loss-Tolerant Quantum Communication via Bosonic-GKP-Parity-Encoding

Abstract: Quantum repeaters constitute a promising platform for enabling long distance quantum communication and may ultimately serve as the backbone of a secure quantum internet, a scalable quantum network, or a distributed quantum computer. An efficient approach to encoding qubits within an error-correcting code is provided by bosonic codes, in which even a single oscillator mode can function as a sufficiently large physical system. In this work, initially we focus on the bosonic Gottesman Kitaev Preskill (GKP) code as a natural candidate for loss correction based quantum repeaters, which can be implemented at room temperature. We demonstrate that transmission loss can be suppressed across three related protocols at the expense of the introduction of logical errors. The third protocol, where a relay-like teleamplifier is applied is optimal. This approach enables medium-distance quantum communication without requiring higher level encoding. We compute the resulting secure key rates while leveraging analog syndrome information. Furthermore, we propose a concatenated Bell state measurement (CBSM) scheme with a modified parity encoding based on GKP qubits, CV measurement and a clipping method that corrects transmission loss without introducing logical errors. This significantly enhances the possible transmission distance. We find that GKP based repeaters can achieve performance comparable to approaches relying on photonic qubits, while requiring orders of magnitude fewer qubits.