Topologically Protected All-Optical Memory

The in-memory processor has played an essential role in overcoming the von Neumann bottleneck, which arises from the partition of memory and a processing unit. Although photonic technologies have recently attracted attention for ultrafast and power-efficient in-memory computing, the realization of an all-optical in-memory processor remains a challenge. This difficulty originates from the contradiction between robustness and sensitivity in wave dynamics, requiring both noise-immune memory states and modulation-sensitive transitions between these states. Here, a building block that provides an all-optical transition between topologically protected memory states is proposed. A nonlinear photonic molecule that satisfies parity-time (PT) symmetry, revealing multiple oscillation quenching states with different degeneracies determined by PT-symmetric phases is investigated. In terms of topology for dynamical systems, these quenching states support topologically protected dynamical trajectories suitable for stable memory states. An all-optical bidirectional transition between these states, which allows incoherent memory switching is demonstrated. The result provides design criteria for all-optical in-memory processors with multilevel operations, enabling the classical-wave counterpart of electronic memristors.