Adaptive optimal control of entangled qubits

Developing fast, robust, and accurate methods for optimal control of quantum systems comprising interacting particles is one of the most active areas of current science. Although a valuable repository of algorithms is avail-able for numerical applications in quantum control, the high computational cost is somewhat overlooked. Here, we present a fast and accurate optimal control algorithm for systems of interacting qubits, QOALA (quantum optimal control by adaptive low-cost algorithm), which is predicted to offer O(M2) speedup for an M-qubit system, compared to the state-of-the-art exact methods, without compromising overall accuracy of the optimal solution. The method is general and compatible with diverse Hamiltonian structures. The proposed approach uses inexpensive low-accuracy approximations of propagators far from the optimum, adaptively switching to higher accuracy, higher-cost propagators when approaching the optimum. In addition, the utiliza-tion of analytical Lie algebraic derivatives that do not require computationally expensive matrix exponential brings even better performance.

Automated summary

Developing fast, robust, and accurate methods for optimal control of quantum systems is crucial in current scientific research. We introduce a fast and accurate algorithm, QOALA, for optimal control of interacting qubit systems, which offers significant speedup compared to existing methods, without sacrificing accuracy.