Continuous-time measurements are instrumental for a multitude of tasks in quantum engineering and quantum control, including the estimation of dynamical parameters of open quantum systems monitored through the environment. However, such measurements do not extract the maximum amount of information available in the output state, so finding alternative optimal measurement strategies is a major open problem. In this paper we solve this problem in the setting of discrete-time input-output quantum Markov chains. We present an efficient algorithm for optimal estimation of one-dimensional dynamical parameters which consists of an iterative procedure for updating a `measurement filter' operator and determining successive measurement bases for the output units. A key ingredient of the scheme is the use of a coherent quantum absorber as a way to post-process the output after the interaction with the system. This is designed adaptively such that the joint system and absorber stationary state is pure at a reference parameter value. The scheme offers an exciting prospect for optimal continuous-time adaptive measurements, but more work is needed to find realistic practical implementations.
翻译:连续时间测量有助于量子工程和量子控制方面的多种任务,包括估计通过环境监测的开放量子系统的动态参数,但是,这种测量并不提取输出状态中可获得的最大数量的信息,因此寻找其他最佳计量战略是一个主要的未决问题。在本文件中,我们在设定离散时间输入输出量量子Markov链时解决了这个问题。我们提出了一个优化估算单维动态参数的有效算法,其中包括更新“测量过滤器”操作器的迭代程序,以及确定输出单位的连续测量基础。这个方法的一个关键组成部分是使用连贯的量子吸收器,作为处理与系统互动后产出后的一种方法。这个方法的设计适应性使得联合系统和吸收器固定状态在参考参数值上是纯净的。这个方法为最佳连续时间适应测量提供了令人振奋的希望,但需要做更多的工作才能找到现实的实际实施。