Jackson Kimball Derek F, Sushkov Alexander O, Budker Dmitry
Department of Physics, California State University-East Bay, Hayward, California 94542-3084, USA.
Department of Physics, Boston University, Boston, Massachusetts 02215, USA.
Phys Rev Lett. 2016 May 13;116(19):190801. doi: 10.1103/PhysRevLett.116.190801.
A ferromagnetic needle is predicted to precess about the magnetic field axis at a Larmor frequency Ω under conditions where its intrinsic spin dominates over its rotational angular momentum, Nℏ≫IΩ (I is the moment of inertia of the needle about the precession axis and N is the number of polarized spins in the needle). In this regime the needle behaves as a gyroscope with spin Nℏ maintained along the easy axis of the needle by the crystalline and shape anisotropy. A precessing ferromagnetic needle is a correlated system of N spins which can be used to measure magnetic fields for long times. In principle, by taking advantage of rapid averaging of quantum uncertainty, the sensitivity of a precessing needle magnetometer can far surpass that of magnetometers based on spin precession of atoms in the gas phase. Under conditions where noise from coupling to the environment is subdominant, the scaling with measurement time t of the quantum- and detection-limited magnetometric sensitivity is t^{-3/2}. The phenomenon of ferromagnetic needle precession may be of particular interest for precision measurements testing fundamental physics.
据预测，在铁磁针的固有自旋主导其旋转角动量的条件下，即(N\hbar\gg I\Omega)（(I)是针绕进动轴的转动惯量，(N)是针中极化自旋的数量），铁磁针会以拉莫尔频率(\Omega)绕磁场轴进动。在这种情况下，针的行为类似于陀螺仪，通过晶体和形状各向异性，自旋(N\hbar)沿针的易轴保持。进动的铁磁针是(N)个自旋的相关系统，可用于长时间测量磁场。原则上，利用量子不确定性的快速平均，进动针式磁力计的灵敏度可以远远超过基于气相中原子自旋进动的磁力计。在与环境耦合产生的噪声占比小的条件下，量子和检测极限磁力计灵敏度随测量时间(t)的缩放比例为(t^{-3/2})。铁磁针进动现象对于测试基础物理学的精密测量可能特别有意义。
