Probing quantum many-body dynamics using subsystem Loschmidt echos

The Loschmidt echo, or return probability, reveals key information about quantum many-body dynamics. However, its exponential decrease with system size limits its practical implementation. In this poster, we will introduce the subsystem Loschmidt echo, a quasi-local observable, as a novel resource efficient and experimentally-accessible observable that captures key feature of the full Loschmidt echo. Using quantum gas microscopy, we measure the subsystem Loschmidt echo of quantum states during quench dynamics. In the short-time regime, we observe a non-analytic, cusp-like feature of the subsystem Loschmidt echo at the dynamical critical point which becomes more pronounced with increasing subsystem size - a hallmark of a dynamical quantum phase transition. Furthermore, we identify that the dynamical quantum phase transition originates from genuine higher-order correlations. In the long-time regime, when the system reaches its quasi-equilibrium state, we quantitatively measure the effective dimension of accessible Hilbert space for both ergodic and kinetically constrained models in the thermodynamic limit. This provides direct experimental evidence of Hilbert space fragmentation due to strong kinetic constraints.