Quantum repeaters based on concatenated bosonic and discrete-variable quantum codes

Abstract

We propose an architecture of quantum-error-correction-based quantum repeaters that combines techniques used in discrete- and continuous-variable quantum information. Specifically, we propose to encode the transmitted qubits in a concatenated code consisting of two levels. On the first level we use a continuous-variable GKP code encoding the qubit in a single bosonic mode. On the second level we use a small discrete-variable code. Such an architecture has two important features. Firstly, errors on each of the two levels are corrected in repeaters of two different types. This enables for achieving performance needed in practical scenarios with a reduced cost with respect to an architecture for which all repeaters are the same. Secondly, the use of continuous-variable GKP code on the lower level generates additional analog information which enhances the error-correcting capabilities of the second-level code such that long-distance communication becomes possible with encodings consisting of only four or seven optical modes.

17 + 26 pages, 17 figures. v2: additional discussions of generation of higher-level-encoded states and of the secret-key rate performance metric; detailed simulation error analysis; restructuring of the sections with the new "Methods" section; published version. See also the related work by Fukui et al., arXiv:2011.14876