Entanglement question

A Reddit user seeks a non-destructive method to verify if a qubit in Tokyo remains entangled with its partner in London, without classical communication or breaking potential entanglement. The challenge requires local-only operations, as the user lacks access to Alice’s qubit in London or any external communication channels to cross-verify entanglement status. Key constraints include preserving any existing entanglement, ignoring qubit state values, and using only currently available scientific equipment for repeated testing. The scenario assumes a pre-existing Bell pair, but the specific type is irrelevant—only detecting entanglement’s presence matters without measuring or collapsing the system. Solutions must rely solely on local quantum operations, such as entanglement witnesses or non-demolition measurements, to infer entanglement status indirectly.