Quantum Nanoelectronics Laboratory, Department of Physics, University of California, Berkeley, California 94720, USA.
Department of Physics and Astronomy and Center for Coherence and Quantum Optics, University of Rochester, Rochester, New York 14627, USA.
Nature. 2014 Jul 31;511(7511):570-3. doi: 10.1038/nature13559.
A central feature of quantum mechanics is that a measurement result is intrinsically probabilistic. Consequently, continuously monitoring a quantum system will randomly perturb its natural unitary evolution. The ability to control a quantum system in the presence of these fluctuations is of increasing importance in quantum information processing and finds application in fields ranging from nuclear magnetic resonance to chemical synthesis. A detailed understanding of this stochastic evolution is essential for the development of optimized control methods. Here we reconstruct the individual quantum trajectories of a superconducting circuit that evolves under the competing influences of continuous weak measurement and Rabi drive. By tracking individual trajectories that evolve between any chosen initial and final states, we can deduce the most probable path through quantum state space. These pre- and post-selected quantum trajectories also reveal the optimal detector signal in the form of a smooth, time-continuous function that connects the desired boundary conditions. Our investigation reveals the rich interplay between measurement dynamics, typically associated with wavefunction collapse, and unitary evolution of the quantum state as described by the Schrödinger equation. These results and the underlying theory, based on a principle of least action, reveal the optimal route from initial to final states, and may inform new quantum control methods for state steering and information processing.
量子力学的一个核心特征是测量结果本质上是概率性的。因此，连续监测量子系统会随机地干扰其自然的幺正演化。在存在这些涨落的情况下控制量子系统的能力在量子信息处理中变得越来越重要，并在从核磁共振到化学合成等领域得到了应用。详细了解这种随机演化对于开发优化控制方法至关重要。在这里，我们重建了在连续弱测量和拉比驱动的竞争影响下演化的超导电路的单个量子轨迹。通过跟踪在任何选定的初始和最终状态之间演化的单个轨迹，我们可以推断出量子态空间中最可能的路径。这些预先和后选择的量子轨迹还揭示了最优探测器信号的形式，即连接所需边界条件的平滑、时连续函数。我们的研究揭示了测量动力学与量子状态的幺正演化之间的丰富相互作用，测量动力学通常与波函数塌缩有关，而量子状态的幺正演化则由薛定谔方程描述。这些结果和基于最小作用原理的基础理论揭示了从初始状态到最终状态的最优路径，并可能为状态引导和信息处理的新量子控制方法提供信息。
