Coherent feedback control of a single qubit in diamond

Area of Science:

• Quantum Information Science
• Solid-State Quantum Systems
• Quantum Control Engineering

Background:

• Controlling quantum systems against environmental noise is crucial for quantum technologies.
• Open-loop control methods like dynamical decoupling are common but have limitations.
• Feedback control, successful in classical systems, is challenging in quantum settings due to measurement complexities.

Purpose of the Study:

• To implement and demonstrate a coherent feedback control algorithm for a solid-state spin qubit.
• To show that this feedback method can protect qubits against dephasing noise.
• To explore the potential of feedback control as an alternative to existing quantum control strategies.

Main Methods:

• Utilized a nitrogen vacancy center in diamond as a solid-state spin qubit system.
• Implemented a coherent feedback loop involving an auxiliary quantum controller (ancilla).
• Employed entangling operations for information acquisition and conditional gates for feedback actions.

Main Results:

• Successfully protected the solid-state spin qubit against intrinsic dephasing noise for milliseconds.
• Demonstrated coherent feedback's ability to protect against Markovian noise, unlike open-loop methods.
• Showcased that feedback control can allow gate operations while providing protection.

Conclusions:

• Coherent feedback control is a viable and effective strategy for protecting quantum information in solid-state systems.
• This method offers advantages over dynamical decoupling and approaches quantum error correction without large qubit overheads.
• The protocol's robustness was evaluated, highlighting a trade-off between information gain and decoherence protection.