Figure 3.

MIT experimental setups for the (a) single-photon, (b) two-photon two-coil, and (c) two-photon single-coil configurations. The gold coloured arrows represent the field that drives the response of interest (low frequency) from the object relevant to the inductive measurements. (a) In the single-photon self-compensation case, the bias field is directed along the primary field ${\mathbf{B}}_{\mathrm{sp}}\left(t\right)$ (double-ended gold arrow), and the magnetometer is sensitive to secondary fields ${\mathbf{B}}_{\mathrm{sp}}^{\prime }\left(t\right)$ in the 2D plane perpendicular to ${\mathbf{B}}_0$. (b) In the two-photon two-coil configuration, the high-frequency auxiliary coil producing ${\mathbf{B}}_1\left(t\right)$ is far from the plate. The low-frequency rf field ${\mathbf{B}}_2\left(t\right)$ can penetrate through the material due to its large skin depth, and the secondary field ${\mathbf{B}}_2^{\prime }\left(t\right)$ induced parallel to the surface of the plate is measured by the sensor. The optimal geometric configuration is chosen due to the $1/{\omega }_2$ amplitude dependence of the two-photon coherence, described in Section 3.3. (c) In the two-photon single-coil case, both frequency components come from the same coil. Only the low-frequency component will produce a secondary field ${\mathbf{B}}_2^{\prime }\left(t\right)$ along the bias field due to the attenuation of the high-frequency rf field at the object’s surface.