Research Laboratory of Electronics and Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA.
Nature. 2016 Apr 7;532(7597):77-80. doi: 10.1038/nature17404.
Engineering desired operations on qubits subjected to the deleterious effects of their environment is a critical task in quantum information processing, quantum simulation and sensing. The most common approach relies on open-loop quantum control techniques, including optimal-control algorithms based on analytical or numerical solutions, Lyapunov design and Hamiltonian engineering. An alternative strategy, inspired by the success of classical control, is feedback control. Because of the complications introduced by quantum measurement, closed-loop control is less pervasive in the quantum setting and, with exceptions, its experimental implementations have been mainly limited to quantum optics experiments. Here we implement a feedback-control algorithm using a solid-state spin qubit system associated with the nitrogen vacancy centre in diamond, using coherent feedback to overcome the limitations of measurement-based feedback, and show that it can protect the qubit against intrinsic dephasing noise for milliseconds. In coherent feedback, the quantum system is connected to an auxiliary quantum controller (ancilla) that acquires information about the output state of the system (by an entangling operation) and performs an appropriate feedback action (by a conditional gate). In contrast to open-loop dynamical decoupling techniques, feedback control can protect the qubit even against Markovian noise and for an arbitrary period of time (limited only by the coherence time of the ancilla), while allowing gate operations. It is thus more closely related to quantum error-correction schemes, although these require larger and increasing qubit overheads. Increasing the number of fresh ancillas enables protection beyond their coherence time. We further evaluate the robustness of the feedback protocol, which could be applied to quantum computation and sensing, by exploring a trade-off between information gain and decoherence protection, as measurement of the ancilla-qubit correlation after the feedback algorithm voids the protection, even if the rest of the dynamics is unchanged.
对处于环境有害影响下的量子位执行所需操作是量子信息处理、量子模拟和传感的关键任务。最常见的方法依赖于开环量子控制技术，包括基于解析或数值解的最优控制算法、李雅普诺夫设计和哈密顿工程。受经典控制成功的启发，另一种策略是反馈控制。由于量子测量带来的复杂性，闭环控制在量子环境中不太普遍，并且除了一些例外，其实验实现主要限于量子光学实验。在这里，我们使用与金刚石中的氮空位中心相关的固态自旋量子位系统实现了一种反馈控制算法，使用相干反馈克服了基于测量的反馈的限制，并表明它可以保护量子位免受内在去相位噪声的影响长达毫秒。在相干反馈中，量子系统与辅助量子控制器（辅助量子比特）相连，该控制器获取系统输出状态的信息（通过纠缠操作）并执行适当的反馈操作（通过条件门）。与开环动态解耦技术不同，反馈控制即使在马科夫噪声和任意时间段内（仅受辅助量子比特的相干时间限制）也可以保护量子位，同时允许门操作。因此，它与量子纠错方案更密切相关，尽管这些方案需要更大和不断增加的量子比特开销。增加新鲜辅助量子比特的数量可以在其相干时间之外提供保护。我们通过探索信息增益和退相干保护之间的权衡，进一步评估反馈协议的鲁棒性，该协议可以应用于量子计算和传感，在反馈算法之后测量辅助量子比特相关性会破坏保护，即使其余动力学不变。
