Accurate and efficient control of quantum systems is one of the central challenges for quantum information processing. Current state-of-the-art experiments rarely go beyond 10 qubits and in most cases demonstrate only limited control. Here we demonstrate control of a 12-qubit system, and show that the system can be employed as a quantum processor to optimize its own control sequence by using measurement-based feedback control (MQFC). The final product is a control sequence for a complex 12-qubit task: preparation of a 12-coherent state. The control sequence is about 10% more accurate than the one generated by the standard (classical) technique, showing that MQFC can correct for unknown imperfections. Apart from demonstrating a high level of control over a relatively large system, our results show that even at the 12-qubit level, a quantum processor can be a useful lab instrument. As an extension of our work, we propose a method for combining the MQFC technique with a twirling protocol, to optimize the control sequence that produces a desired Clifford gate.
@article{arxiv.1701.01198,
title = {Enhancing quantum control by bootstrapping a quantum processor of 12 qubits},
author = {Dawei Lu and Keren Li and Jun Li and Hemant Katiyar and Annie Jihyun Park and Guanru Feng and Tao Xin and Hang Li and GuiLu Long and Aharon Brodutch and Jonathan Baugh and Bei Zeng and Raymond Laflamme},
journal= {arXiv preprint arXiv:1701.01198},
year = {2017}
}