  We employ strange correlators to detect 2D subsystem symmetry-protected
topological (SSPT) phases which are nontrivial topological phases protected by
subsystem symmetries. Specifically, we analytically construct efficient strange
correlators in the 2D cluster model in the presence of a uniform magnetic field
and then perform the projector Quantum Monte Carlo simulation within the
quantum annealing scheme. We find that strange correlators show the long-range
correlation in the SSPT phase, from which we define strange order parameters to
characterize the topological phase transition between the SSPT phase at low
fields and the trivial paramagnetic phase at high fields. Thus, the detection
of the fully localized zero modes on the 1D physical boundary of SSPT phase has
been transformed into the bulk correlation measurement about the local
operators with the periodic boundary condition. We also find interesting
spatial anisotropy of a strange correlator, which can be intrinsically traced
back to the nature of spatial anisotropy of subsystem symmetries that protect
SSPT order in the 2D cluster model. By simulating strange correlators, we,
therefore, provide the first unbiased large-scale quantum Monte Carlo
simulation on the easy and efficient detection in the SSPT phase and open the
avenue of the investigation of the subtle yet fundamental nature of the novel
interacting topological phases.
