Autonomous stabilization of remote entanglement in a cascaded quantum network (Part 2: Detecting and optimizing entanglement)

Remote entanglement between widely separated qubits is a fundamental quantum phenomenon and a critical resource for quantum information applications. Here, we report autonomous stabilization of entanglement between two separate superconducting-qubit devices. Combining nonreciprocal waveguide coupling and local driving, we experimentally realize distance-independent steady-state remote entanglement in a coherent quantum-absorber (CQA) scheme. In Part-II of this talk, we discuss how mismatched qubit-waveguide couplings violate the symmetry requirement of the CQA scheme, resulting in a degree of stabilized entanglement that falls short of what can be explained by the loss rates of the system. We overcome this challenge by identifying a connection to a protocol that mimics two-mode squeezing and is robust to arbitrary degrees of qubit-waveguide coupling mismatch. We use this analogy to derive optimal drive conditions which result in a concurrence approaching 0.5. Our results set the stage for on-demand entanglement delivery in quantum processors and networks, and for protecting multipartite entanglement in open systems.