Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells

Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multipass optical cells. We use a pump-probe measurement scheme to suppress spin-exchange relaxation and two probe pulses to find the spin precession zero crossing times with a resolution of 1 psec. We realize a magnetic field sensitivity of $0.54\mathrm{fT}/{\mathrm{Hz}}^{1/2}$, which improves by an order of magnitude the best scalar magnetometer sensitivity and exceeds, for example, the quantum limit set by the spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime.

Figure 1

(a) Experiment setup. PBS: polarization beam splitter; PMF: polarization maintaining fiber; DAQ: data acquisition card. (b) The timing of the pulsed operation. (c) Optical rotation (black line) recorded for one probe pulse at atom density of $0.8\times {10}^{13}/{\mathrm{cm}}^3$ together with a fitted curve (red dashed line). (d) Magnetic field noise spectrum obtained in the gradiometer in the presence of a calibrating magnetic field gradient at 40 Hz. The peak at 60 Hz is due to ac line noise.

Figure 2

(a) Unpolarized atom spin noise spectrum taken at 0.42 G with atom density of $1.2\times {10}^{13}/{\mathrm{cm}}^3$ after subtraction of the photon shot noise background from a spin noise measurement at low magnetic field. (b) Spin correlation function due to the diffusion of the atoms out of the probe beam. Solid line (dashed line) is the experimental (calculated) result with atom density of $1.2\times {10}^{13}/{\mathrm{cm}}^3$. The inset shows the calculated beam pattern at the center of the cavity.

Figure 3

The dependence of the standard deviation $\delta T_c$ on the rf pulse amplitude. Empty (solid) circles denote the backaction evasion with stroboscopic modulation (normal modulation) of probe light. Here the probe pulse length and $T$ are equal to four Larmor periods.

Figure 4

(a) Relaxation of transverse polarization $P_t$ (points) for $n=1.9$ (green), 1.4 (blue), 0.8 (red), and $0.6(\mathrm{\text{black}})\times {10}^{13}/{\mathrm{cm}}^3$ together with theoretical prediction (lines). (b) The points are the experimental results of magnetic field sensitivity at $n=1.4\times {10}^{13}/{\mathrm{cm}}^3$, with probe pulse length of 4 (orange), 8 (cyan), and 12 (purple) Larmor periods. Solid (dashed) lines are the theoretically calculated sensitivity based on measured parameters with (without) magnetic shield noise and time jitter noise. (c) Experiment results (points) of magnetic field sensitivity with probe length of 12 Larmor periods at different atom densities, with the same color notation as plot (a), together with theoretical predictions (lines).