Autonomous stabilization of remote entanglement in a cascaded quantum network (Part 1: Network characterization and quantum state reconstruction)

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 scheme (CQA). In Part-I of this talk, we discuss the design and characterization of a low-loss cascaded network of two separated superconducting-qubit devices. A crucial difficulty in implementing this protocol results from the radiative coupling of the qubit to the waveguide: the short qubit lifetimes result in imperfect single-qubit gates, which makes inferring the quantum state nontrivial. To overcome this challenge, we perform quantum detector tomography (QDT) using a set of known states and their measurement outcomes. This procedure characterizes the positive operator-valued measures (POVMs) that characterize the measurements, and their knowledge allows us to reconstruct the two-qubit quantum state.