Curator's Take
AI Commentary
This article demonstrates the first experimental observation of mixed‑state symmetry‑protected topological (SPT) order and its transformation under measurement and controlled dephasing in a five‑qubit superconducting array, showing that distinct symmetry sectors can survive decoherence while others are selectively suppressed. By conditioning on measurement outcomes and using nonlinear fidelity correlators, the team reveals a finite‑size signature of strong‑to‑weak spontaneous symmetry breaking—a phenomenon that bridges recent advances in measurement‑based quantum computing and topological error protection. Although limited to a small register, the work establishes superconducting circuits as a versatile platform for probing hidden order and could inform future designs of robust quantum memories or sensors that exploit mixed‑state topology.
— Mark Eatherly
Summary
We experimentally investigate how symmetry-protected topological order in a one-dimensional cluster state is transformed by measurement and decoherence in a five-qubit superconducting array. We first characterize the state's nonlocal string order and show that controlled dephasing selectively suppresses one symmetry sector while leaving the other robust, consistent with average symmetry-protected topological order. We then measure one sublattice in a tunable basis and show that the remaining qubits are driven between a long-range-entangled GHZ state and a paramagnetic state. When the measurement record is discarded, the conventional long-range correlator vanishes while a nonlinear fidelity correlator remains finite, providing a finite-size signature of strong-to-weak spontaneous symmetry breaking. These experiments demonstrate how conditioning, averaging, and decoherence reveal distinct manifestations of order encoded in the same underlying cluster state, and establish a superconducting-circuit setting for probing mixed-state symmetry and topology.