Magnetometry

BEC

Spin-echo-based magnetometry with spinor Bose-Einstein condensates

Abstract. In this work they have noticed that magnetic field created by ac line disrupts their experiments, by applying a magnetic field with an opposite phase they reduce the noise to 1 nT. As a magnetometer they achieved 12 pT/sqrt(Hz) with spatial resolution 100 um^2. For the experiment they average over 10 points.

Technicalities. For the coils they use low ripple current source <2uA (Newport 505), they insert permalloy plates into the room walls. The BEC of ⁸⁷Rb is prepared in a crossed far-off resonant optical trap (FORT with w_axial=30 Hz, w_rad=100 Hz), 5*10³ atoms are prepared in F=2.

Idea. The rf Hahn-echo pulse sequence is sent: $\pi/2\rightarrow\pi\rightarrow\pi/2$ If there is an external varying magnetic field the phase measured in this experiment is: $\theta = \frac{g_F \mu_B}{\hbar} \left[ \int_0^{\tau/2}b(t)dt- \int_{\tau/2}^{\tau}b(t)dt \right]$. The sensitivity of the magnetometer for a single measurement is given by: $\frac{\delta<S_z>}{d<S_z>/db_{ac}}$, where ${\delta<S_z>}$ is uncertainty of a single measurement.

One more trick to raise the accuracy is to spatially separate atoms with different m_F values, thus to obtain all of them in one measurement.