Closing a quantum feedback loop inside a cryostat: Autonomous state preparation and long-time memory of a superconducting qubit

Understand

We propose to use a cryogenic nonlinear resonator for the projective readout, classical memory, and feedback for a superconducting qubit.

• This approach sidesteps many of the inefficiencies inherent in two-way communication between temperature stages in typical systems with room temperature controllers, and avoids increasing the cryogenic heat load.
• This controller may find a broad range of uses in multi-qubit systems, but here we analyze two specific demonstrative cases in single qubit-control.
• In the first case, the nonlinear controller is used to initialize the qubit in a definite eigenstate.