Robustness of Bell violation of graph states to qubit loss

Abstract

Graph states are special entangled states advantageous for many quantum technologies, including quantum error correction, multiparty quantum communication, and measurement-based quantum computation. Yet, their fidelity is often disrupted by various errors, most notably qubit loss. In general, given an entangled state, Bell inequalities can be used to certify whether quantum entanglement remains despite errors. Here, we study the robustness of graph states to loss in terms of their Bell violation. Considering the recently proposed linearly scalable Bell operators by Baccari et al. [Phys. Rev. Lett. 124, 020402 (2020)PRLTAO0031-900710.1103/PhysRevLett.124.020402], we use the stabilizer formalism to derive a formula for the extent by which the Bell violation of a given graph state is decreased with qubit loss. Our analysis allows one to determine which graph topologies are tolerable to qubit loss as well as pinpointing the Achilles' heel of each graph, namely the sets of qubits whose loss jeopardizes the Bell violation. Our results serve as an analytical tool for optimizing experiments and protocols involving graph states in realistically lossy systems. An experimental demonstration of a Bell violation in our noise-tolerant graphs is within reach using state of the art technology.