In quantum feedback measurements, the results of single-shot qubit measurements are used as decision input for an immediate feedback action on the qubits. The lower the feedback latency, the smaller the error rate of the feedback operation and the higher the fidelity of the overall quantum information processing operation. To ensure repeatability, the entire feedback loop needs to be completed with deterministic timing even when passing through multiple instruments. Quantum feedback is used in applications such as rapid qubit initialization, quantum state stabilization, and quantum error correction. Use cases differ in the complexity of the required signal processing between measurement and feedback; the signal processing step ranges from a simple forwarding of digital bits of information to demanding error syndrome decoding.

Zurich Instruments' products cover the full range of configurations required in superconducting and spin qubit experiments to make sure that the best trade-off between feedback speed and complexity handling is achieved.

A Zurich Instruments Quantum Computing Control System (QCCS) of the first generation relies on qubit control and readout signals generated in the baseband; the QCCS of the second generation operates directly at microwave frequencies up to 8.5 GHz. Both generations of instruments support the same feedback methods, but with differences in the implementation. In the following, we describe the possibilities and implementations for both generations.