The ultimate precision limit in estimating the Larmor frequency of N unentangled qubits is well established, and is highly important for magnetometers, gyroscopes, and other types of quantum sensors. However, this limit assumes perfect projective measurements of the quantum registers. This requirement is not practical in many physical systems, such as NMR spectroscopy, where a weakly interacting external probe is used as a measurement device. Here, we show that in the framework of quantum nano-NMR spectroscopy, in which these limitations are inherent, the ultimate precision limit is still achievable using control and a finely tuned measurement.

中文翻译：

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通过弱相互作用的量子探针达到极限精度

估算N个无纠缠量子比特的拉莫尔频率的最终精度极限已得到充分确立，并且对于磁力计，陀螺仪和其他类型的量子传感器非常重要。但是，此限制假设量子寄存器的完美投影测量。此要求在许多物理系统（例如NMR光谱）中不切实际，在该系统中，弱相互作用的外部探针用作测量设备。在这里，我们表明，在固有的这些局限性的量子纳米NMR光谱学框架内，使用控制和微调的测量仍然可以达到最终的精度极限。