比利时列日大学John Martin团队研究了混合自旋态旋转的量子计量。2025年2月20日，《物理评论A》杂志发表了这一成果。

量子计量协议的效率会因系统与其环境的相互作用而显著降低，导致纯度损失，从而导致探测系统处于混合状态。一个例子是通过自旋的旋转来测量磁场，因其与周围的自旋或玻色子浴耦合，自旋会发生退相干。

在这项工作中，研究组将混合最优量子旋转传感器（OQR）定义为混合自旋j态，当旋转轴未知时，可实现估计无穷小旋转的最大灵敏度。他们研究了两种情况，其中探测态饱和了平均保真度或平均量子Cramér-Rao界，后者给出了最终的灵敏度极限。

研究组发现混合OQR可实现与纯态相同的灵敏度，并且是通过从反相干态的线性子空间混合态获得的。他们给出了混合OQR及其相关反相干子空间的几个例子。研究组还表明，OQR在特定意义上最大化了纠缠，即使在混合态的情况下，也保持了纯态纠缠与最佳旋转灵敏度之间的已知关系。该研究结果强调了混合自旋态中旋转、反相干和纠缠的量子计量之间的相互联系。

附：英文原文

Title: Quantum metrology of rotations with mixed spin states

Author: Eduardo Serrano-Ensástiga, Chryssomalis Chryssomalakos, John Martin

Issue&Volume: 2025/02/20

Abstract: The efficiency of a quantum metrology protocol can be significantly diminished by the interaction of the system with its environment, leading to a loss of purity and, as a result, a mixed state for the probing system. An example is the measurement of a magnetic field through the rotation of a spin that is subject to decoherence due to its coupling to a surrounding spin or bosonic bath. In this work we define mixed optimal quantum rotosensors (OQRs) as mixed spin- states that achieve maximum sensitivity to estimate infinitesimal rotations, when the rotation axis is unknown. We study two scenarios, where the probe states saturate the averaged fidelity or the averaged quantum Cramér-Rao bound, the latter giving the ultimate sensitivity. We find that mixed OQRs can achieve sensitivity equal to that of pure states and are obtained by mixing states from linear subspaces of anticoherent states. We present several examples of mixed OQRs and their associated anticoherent subspaces. We also show that OQRs maximize entanglement in a specific sense, preserving the known relation between entanglement and optimal rotation sensitivity for pure states, even in the context of mixed states. Our results highlight the interconnection between quantum metrology of rotations, anticoherence, and entanglement in mixed spin states.

DOI: 10.1103/PhysRevA.111.022435

Source: https://journals.aps.org/pra/abstract/10.1103/PhysRevA.111.022435