Globally stabilizing two-qubit entanglement with dynamically decoupled active feedback*

We propose and analyze a protocol for stabilizing a maximally entangled state of two noninteracting qubits using active state-dependent feedback from a continuous two-qubit half-parity measurement. We implement techniques for accurately modeling continuous, state-dependent feedback and introduce a forward-state-estimation strategy in the feedback controller that tracks the effects of control signals already in transit. 

 

We demonstrate that, depending on the noise spectrum, the optimal protocol includes a dynamical decoupling drive that is simultaneous with the measurement and feedback and that also serves a key role in the feedback cycle by removing a unwanted fixed point of the dynamics.

 

Our results demonstrate robust stabilization with near-unit fidelity even in the presence of realistic nonidealities, such as time delay in the feedback loop, imperfect state-tracking, inefficient measurements, dephasing from 1/f-distributed qubit-frequency noise, and T1 decay. We compare this global stabilization to a previously studied protocol, which in the presence of the same non-idealities leads to near zero steady-state entanglement.