Indirect control of qubits by a quantum actuator is an attractive strategy in many situations when the qubits couple weakly to external fields, but interact more strongly with another quantum system. For electronic and nuclear spins (qubits) centered around the NV center (actuator) in diamond, we analyzed when indirect control presents a time advantage over direct control methods.

Time-optimal control by a quantum actuator

Indirect control of qubits by a quantum actuator has been proposed as an appealing strategy to manipulate qubits that couple only weakly to external fields. While universal quantum control can be easily achieved when the actuator-qubit coupling is anisotropic, the efficiency of this approach is less clear.

Here we analyze the time efficiency of quantum actuator control. We describe a strategy to find time-optimal control sequences by the quantum actuator and compare their gate times with direct driving, identifying regimes where the actuator control performs faster.

As a paradigmatic example, we focus on a specific implementation based on the nitrogen-vacancy center electronic spin in diamond (the actuator) and nearby $^{13}$C nuclear spins (the qubits).

Physical Review A 91 042340