[论文解读] Back-action-free measurement of a single nuclear spin

Matthias Pfender, Ping Wang+9 more|arXiv (Cornell University)|Jun 6, 2018

Quantum and electron transport phenomena被引用 1

一句话总结

该论文通过将测量时间与自旋进动同步，实现了对单个核自旋的无反作用力量子测量，实现了100毫秒的无扰动量子演化和3.4 Hz的频率分辨率。该方法使精度传感达到量子反作用极限，且在强测量条件下观察到向相干自旋捕获的相变。

ABSTRACT

The precision of any measurement is ultimately limited by quantum mechanics, due to the intrinsic back action of quantum measurements. This is particularly relevant for nanoscale precision measurements where the sensor and the target to be measured are both quantum objects in close contact. A potential solution is to use weak measurements, where one tries to mitigate back action by minimally entangling sensor and probe. While weak measurements have been studied extensively in theory and demonstrated experimentally in superconducting qubits and other systems, practical quantum sensing has not yet reached the back action limit. Here, we achieve quantum sensing at the ultimate precision limit set by the measurement strength and demonstrate back-action free precision quantum measurement of a single nuclear spin. By carefully timing the quantum measurement to the precession of the single nuclear spin we reach a regime where the measurement back action is negligible, yielding quantum evolution of the target nuclear spin unperturbed by the measurement over a timescale of 100 ms and hence a frequency resolution of 3.4 Hz. Upon increasing the measurement strength we observe a phase transition to a state where the nuclear spin is coherently trapped during the measurement process. We expect the findings to be of pivotal importance for single molecule NMR and nuclear spin based quantum computing.

研究动机与目标

• 为克服纳米尺度量子测量中的根本性量子反作用限制。
• 在由测量强度决定的最终反作用极限下实现高精度量子传感。
• 展示目标核自旋在测量过程中无扰动地进行量子演化。
• 探索强测量条件下向相干自旋捕获的转变。

提出的方法

• 通过将测量与单个核自旋的进动同步，以最小化反作用。
• 采用弱测量以最小化传感器与目标之间的纠缠，从而减少扰动。
• 精确控制测量时机，使其与核自旋的动力学周期对齐。
• 在100毫秒内监测量子态演化，以评估无扰动动力学。
• 逐步增加测量强度，以观察向相干自旋捕获的转变。
• 使用能够隔离反作用效应的量子传感协议对系统进行分析。

实验结果

研究问题

• RQ1 能否在不产生反作用的情况下对单个核自旋进行量子测量，从而保持其量子演化？
• RQ2 在测量过程中，核自旋能保持无扰动状态的最大时间尺度是多少？
• RQ3 增加测量强度如何影响目标自旋的相干性和动力学？
• RQ4 在强测量条件下，是否会发生向相干自旋捕获的相变？
• RQ5 该方法能否达到由测量强度决定的根本精度极限？

主要发现

• 核自旋在长达100毫秒的时间内未受测量扰动，证明了无反作用力量子传感。
• 实现了3.4 Hz的频率分辨率，接近最终的量子极限。
• 当测量强度增加时，系统表现出向相干自旋捕获态的相变。
• 当测量时机与自旋进动同步时，测量反作用可忽略不计，从而实现高保真度传感。
• 该方法实现了处于根本反作用极限的量子传感，为精度设定了新基准。
• 这些发现预计对单分子NMR和基于核自旋的量子计算具有关键意义。