The continuous fuzzy measurement of energy of a single two-level system driven by a resonant external field is studied. An analysis is given in the framework of the phenomenological restricted path integral approach (RPI) (which reduces effectively to a Schrodinger equation with a complex Hamiltonian) as well as with reference to the microphysical details of a class of concrete physical realizations. Within the RPI approach it is demonstrated that for appropriately adjusted fuzziness, information about the evolution of the state of the system can be read off from the measurement readout E(t). It is shown furthermore how a measurement of this type may be realized by a series of weak and short interactions of the two-level system with a quantum mechanical meter system. After each interaction a macroscopic measuring apparatus causes the meter to transit into one of two states. The result is used to generate the energy readout E(t). In this way a complete agreement with the RPI approach is demonstrated which thus obtains an operational interpretation.
Distinct Feedback-Strength Requirements for Quantum-State Ensemble Preparation under Channel-Equivalent Monitoring
School of Computer Science and Engineering, Macau University of Science and Technology（澳门科技大学计算机科学与工程学院）；Zhuhai MUST Science and Technology Research Institute（珠海澳门科技大学研究院）；School of Information and Software Engineering, University of Electronic Science and Technology of China（电子科技大学信息与软件工程学院）
Princeton University（普林斯顿大学）；Laboratoire de Physique de l’École Normale Supérieure, CNRS, ENS & Université PSL（巴黎高等师范学院物理实验室，法国国家科学研究中心，巴黎文理研究大学）；University of Geneva（日内瓦大学）
Realization of Measurement and the Standard Quantum Limit
本文继[M. Ozawa，Phys. Rev. Lett. 60，385（1988）]，报道了一个反驳的说法，即监测的自由质量，如引力波干涉仪的位置的灵敏度是有限的，由所谓的标准量子极限（SQL）由于不确定原理。分析了SQL的最新证明，揭示了一个不受支持的关于量子测量的假设。引入量子测量理论，给出了量子力学中物理可实现测量的一般判据。近似的位置测量理论的发展，以获得严格的条件SQL，也表明，一个精确的位置测量可以离开对象在一个任意的家庭的状态独立的输入状态。这就得出结论，Yuen提出的通过收缩状态测量来打破SQL的建议，即测量离开自由质量的位置，使其处于位置不确定性随时间减小的状态，原则上是物理上可实现的。为了加强这一结论，一个无误差的位置测量模型，使对象在一个收缩状态构造与测量相互作用的可解哈密顿量。最后，该模型被证明可以以任意精度破坏SQL。
