FET: Small: Weak and Continuous Quantum Measurements with Feedback

量子力学最奇怪的方面之一是量子测量。量子测量并不是简单地揭示系统的信息，而是对系统的状态产生不可预测的扰动，因此不可能在不扰动系统的情况下获得关于量子系统的信息。有一些微弱的测量引起的扰动很小，但这些测量必然也揭示了很少的信息。一个接一个的弱测量序列产生连续的测量，其中信息和干扰都随着时间的推移而积累。该项目涉及弱和连续的量子测量，并将它们与反馈相结合，其中积累的信息用于在测量过程中调整测量。这些方法在量子计算机、高精度测量（或计量学）和其他量子技术中有许多应用。他们也为理解量子理论提供了强有力的理论工具。该奖项将研究弱和连续量子测量的许多方面，但将特别关注三个领域。1)破坏性（或有损）弱测量：许多物理系统-如光-除了被吸收外，不容易被测量。目前的技术只包括一组有限的光量子测量;任何扩展这一组的东西都将是一个令人兴奋的发展。2)连续量子纠错：量子计算机对噪声或退相干非常敏感。这些技术将持续监控量子计算机，并在错误发生时逆转错误，甚至可能通过量子芝诺效应来防止错误。3)量子资源理论：资源理论研究量子系统的特性，如纠缠、相干和自由能，这些特性可用于有用的任务。连续测量理论可以使用单调性来量化资源，单调性随着资源的消耗而稳定地减少或增加，类似于热力学第二定律中熵随时间增长的方式。这个奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

One of the strangest aspects of quantum mechanics is quantum measurement. Rather than simply revealing information about a system, a quantum measurement causes an unpredictable disturbance in the system's state, and it is impossible to gain information about a quantum system without disturbing it. There are weak measurements that cause little disturbance, but these must inevitably also reveal very little information. A sequence of weak measurements, one after another, produces a continuous measurement, where both information and disturbance accumulate over time. This project concerns weak and continuous quantum measurements, and combines them with feedback, where the accumulated information is used to adapt the measurements while they are being done. These methods have many applications, to quantum computers, high-precision measurements (or metrology) and other quantum technologies. They also provide a powerful theoretical tool for understanding quantum theory.This award will study many aspects of weak and continuous quantum measurements, but will focus on three areas in particular. 1) Destructive (or lossy) weak measurements: many physical systems---such as light---cannot be easily measured except by being absorbed. Current technology includes only a limited set of quantum measurements of light; anything that extends this set would be an exciting development. 2) Continuous quantum error correction: quantum computers are highly susceptible to noise, or decoherence. These techniques would continuously monitor a quantum computer and reverse errors as they occur, perhaps even preventing them by the quantum Zeno effect. 3) Quantum resource theories: resource theories study properties of quantum systems, like entanglement, coherence and free energy, that can be exploited for useful tasks. Continuous measurement theory can quantify resources using monotones, which diminish or increase steadily as the resource is consumed, analogous to how entropy grows with time in the second law of thermodynamics. Many more interesting properties and uses of weak quantum measurements are expected to appear in the course of this project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.