宇称-时间对称性破缺相位下的稳定原子磁强计
Stable Atomic Magnetometer in Parity-Time Symmetry Broken Phase
摘要：
自旋的随机运动在磁共振实验中通常是有害的。非均匀磁场中的自旋扩散导致共振的加宽，并限制了如磁共振光谱学的应用中的灵敏度和光谱分辨率。通过对梯度磁场中扩散自旋宇称-时间（$PT$）相变的观测，我们发现原子的空间自由度可以成为高精度测量微弱信号的一种资源，而不是有害的。在正常相，零或低梯度场，扩散导致自旋进动耗散。然而，通过增加场梯度，自旋系统经历$PT$转变，并进入$PT$对称性破缺相。在这个新的阶段，自旋进动频率分裂由于空间本地化的本征模式。我们证明，使用这些空间运动引起的分裂频率，自旋系统可以作为一个稳定的磁强计，其输出是不敏感的控制参数的不可避免的长期漂移。这为在不完全受控的环境中检测极其微弱的信号打开了一扇大门。
英文摘要：
Random motion of spins is usually detrimental in magnetic resonance experiments. The spin diffusion in non-uniform magnetic fields causes broadening of the resonance and limits the sensitivity and the spectral resolution in applications like magnetic resonance spectroscopy. Here, by observation of the parity-time ($PT$) phase transition of diffusive spins in gradient magnetic fields, we show that the spatial degrees of freedom of atoms could become a resource, rather than harmfulness, for high-precision measurement of weak signals. In the normal phase with zero or low gradient fields, the diffusion results in dissipation of spin precession. However, by increasing the field gradient, the spin system undergoes a $PT$ transition, and enters the $PT$ symmetry broken phase. In this novel phase, the spin precession frequency splits due to spatial localization of the eigenmodes. We demonstrate that, using these spatial-motion-induced split frequencies, the spin system can serve as a stable magnetometer, whose output is insensitive to the inevitable long-term drift of control parameters. This opens a door to detect extremely weak signals in imperfectly controlled environment.
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