Collaborative Research: RUI: Weak and Protective Measurements in the Time Domain

该项目将实现测量量子系统的新方法。在物理学中，测量的概念是极其重要的。我们只能通过测量来了解我们周围的世界，例如，当你用眼睛看着某个东西时。在经典物理学中，测量可以在不受干扰的情况下进行--一个人可以在不改变物体状态的情况下获得关于它的信息。(当你看着棒球的轨迹是不变的)。然而，在量子力学中，长期以来人们认为测量几乎总是会改变系统的状态，波函数会“崩溃”，这是没有办法的。最近，这种信念受到了挑战，因为物理学家已经意识到，有可能进行所谓的弱测量或保护性测量，即在获得关于物体的信息的同时，只对其状态进行最小程度的干扰。这种类型的测量正在改变量子测量的格局。它们还在量子信息处理中提供了重要的技术应用。在这个项目中，该小组将实施新的实验技术，以执行微弱和保护性的测量。与现有的实验相比，这些技术有望在测量性能和灵活性方面提供显著改进。同时，该小组将通过理论研究促进我们对微弱和保护性测量的理解，其预测将反过来通过实验进行验证。大部分工作将由不同的本科生完成，为他们提供发展研究技能和进一步发展科学事业的绝佳机会。该项目将在实验上实现单光子微弱和保护性测量，并用理论研究补充这些实验。该实验方法的特点是使用偏振分量之间的时间差分群延迟(DGD)作为测量“指针”。该小组将在电信C波段(~1550 nm)构建一个窄带纯态单光子源，并使用它在DGD后直接测量光子到达时间，从而首次实现偏振的微弱测量。这一基本设置将作为通过在环路配置中利用DGD来测量光子偏振的期望值来实现在时间域中的保护性测量的新的实验实现的中心构建块。光学环路将使团队能够重复通过相同的测量和状态保护阶段的光子，从而实现保护性测量。在该项目的理论部分，该小组将研究保护测量的几个重要性质及其性能，包括：态纯度降低对测量的影响、光子存活概率以及最终测量的不确定度。这些理论活动将产生可通过实验检验的模型和预测，这将增强我们对保护措施的理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

This project will realize new ways of measuring a quantum system. In physics, the concept of a measurement is extremely important. We can only learn about the world around us by performing measurements, e.g., when you look at something you are “measuring" it with your eye. In classical physics, measurements can be performed without disturbance—one gains information about an object without changing its state. (The trajectory of a baseball is unchanged as you look at it). In quantum mechanics, however, it was long believed that a measurement would nearly always change the state of the system, the wave function would “collapse," and there was no way around that fact. Recently that belief has been challenged, as physicists have come to realize that it is possible to perform so-called weak or protective measurements in which one gains information about the object while only minimally disturbing its state. Such types of measurements have been transforming the landscape of quantum measurements. They also offer technologically important applications in quantum information processing. In this project, the group will implement new experimental techniques for performing weak and protective measurements. These techniques promise to provide significant improvements in measurement performance and flexibility over existing experiments. In parallel, the group will advance our understanding of weak and protective measurements through theoretical investigations, whose predictions will in turn be tested by the experiments. Much of the work will be performed by a diverse group of undergraduate students, providing an excellent opportunity for them to develop research skills and further their scientific careers.This project will experimentally realize weak and protective measurements with single photons and complement these experiments with theoretical studies. The experimental approach is characterized by using the temporal differential group delay (DGD) between polarization components as a measuring “pointer." The group will construct a source of narrow-band, pure-state, single photons in the telecommunications C-band (~1550 nm) and use it to first implement a weak measurement of polarization by directly measuring photon arrival times after the DGD. This basic setup will then serve as the central building block of a novel experimental realization of a protective measurement in the time domain by utilizing DGD in a loop configuration to measure expectation values of photon polarization. The optical loop will enable the team to pass the photons repeatedly through the same measurement and state-protection stage, thus implementing the protective measurement. In the theoretical portion of the project, the group will study several important properties of protective measurements and their performance, including: the effect of diminished state purity on the measurement, photon survival probabilities, and the uncertainty in the final measurement. These theoretical activities will generate models and predictions that can be tested by experiment and that will enhance our understanding of protective measurements.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.