Quantum error detection in qubit-resonator star architecture

Abstract

Achieving industrial quantum advantage is unlikely without the use of quantum error correction (QEC). Other QEC codes beyond surface code are being experimentally studied, such as color codes and quantum Low-Density Parity Check (qLDPC) codes, that could benefit from new quantum processing unit (QPU) architectures. We introduce the six-qubit star lattice architecture that offers parallelism and effective local all-to-all connectivity and thus enables hardware-efficient implementation of certain QEC codes. As a first demonstration of this new architecture, we encode two logical qubits in a six-qubit superconducting QPU with a star-topology using the [[4, 2, 2]] code and characterize the logical states with the classical shadow framework. Logical life-time and logical error rate are measured over repeated quantum error detection cycles for various logical states including a logical Bell state. We measure logical state fidelities above 96 % for every cardinal logical state, find logical life-times above the best physical element, and logical error-per-cycle values ranging from 0.25(2) % to 0.91(3) %. In future, such star QPU can be tiled to enable QEC codes with high-weight and overlapping stabilizers for improved encoding rates.