Efficient Detection of Strong-to-Weak Spontaneous Symmetry Breaking via the Rényi-1 Correlator

Strong-to-weak spontaneous symmetry breaking (SWSSB) has recently emerged as a universal feature of quantum mixed-state phases of matter. While various information-theoretic diagnostics have been proposed to define and characterize SWSSB phases, relating these diagnostics to observables which can be efficiently and scalably probed on modern quantum devices remains challenging. Here we propose a new observable for SWSSB in mixed states, called the Rényi-1 correlator, which naturally suggests a route toward scalably detecting certain SWSSB phases in experiment. Specifically, if the canonical purification (CP) of a given mixed state can be reliably prepared, then SWSSB in the mixed state can be detected via ordinary two-point correlation functions in the CP state. We discuss several simple examples of CP states which can be efficiently prepared on quantum devices, and whose reduced density matrices exhibit SWSSB. The Rényi-1 correlator also satisfies several useful theoretical properties: it naturally inherits a stability theorem recently proven for the closely related fidelity correlator, and it directly defines SWSSB as a particular pattern of ordinary spontaneous symmetry breaking in the CP state.