NMR Detection with an Atomic Magnetometer

We demonstrate detection of NMR signals using a noncryogenic atomic magnetometer and describe several novel applications of this technique. A nuclear spin-precession signal from water is detected using a spin-exchange-relaxation-free potassium magnetometer. We also demonstrate detection of less than ${10}^{13}$ ${}^{129}\mathrm{X}\mathrm{e}$ atoms whose NMR signal is enhanced by a factor of 540 due to Fermi-contact interaction with K atoms. The possibility of using a multichannel atomic magnetometer for fast 3D magnetic resonance imaging is also discussed.

Figure 1

Water NMR detection. (a) Experimental setup: tap water is thermally polarized by passing through a 1.4 kG permanent magnet before flowing into a 2.5 cm diameter cylinder located near the magnetometer inside magnetic shields. Pump and probe laser beams pass through evacuated glass tubes to avoid air turbulence. The K cell is a 3.8 cm dia. glass cylinder containing 2.5 atm of He gas and 60 torr of ${\mathrm{N}}_2$ gas and a small droplet of K metal. (b) Single-shot NMR signal excited with a $\pi /2$ pulse and filtered with a bandwidth of 20 Hz. From the fit (dashed line) we determine $T_2^{*}=1.7\sec $. (c) FFT of the NMR signal. The magnetic noise has a flat spectrum with a noise floor of $2\times {10}^{-14}\mathrm{T}/{\mathrm{Hz}}^{1/2}$.

Figure 2

Detection of ${}^{129}\mathrm{X}\mathrm{e}$ NMR signal. (a) Schematic of the experiment and 3 steps of signal detection: polarization of ${}^{129}\mathrm{X}\mathrm{e}$ by spin exchange, tipping of ${}^{129}\mathrm{X}\mathrm{e}$ spins with a constant field, and precession of ${}^{129}\mathrm{X}\mathrm{e}$ spins around the K magnetization. (b) A single ${}^{129}\mathrm{X}\mathrm{e}$ spin-precession signal following the tipping pulse at $t=0$ with a fit (dashed line). From the fit, the initial amplitude of the signal at $t=0$ is 11.4 pT, the frequency is 2.46 Hz, and the transverse relaxation time $T_2^{*}=0.78\sec $. (c) Measurement of $T_1$ as a function of K density. The slope of the fit gives the K-Xe spin-exchange rate ${\sigma }_{\mathrm{SE}}\overline{v}=3.6(9)\times {10}^{-16}{\mathrm{s}}^{-1}{\mathrm{cm}}^{-3}$ and the intercept gives the wall relaxation rate $T_{1\mathrm{wall}}^{-1}=0.016(3){\mathrm{s}}^{-1}$. (d) The ${}^{129}\mathrm{X}\mathrm{e}$ spin-precession frequency as a function of K density. The slope of the fit $8.4(3)\times {10}^{-14}\mathrm{Hz}/{\mathrm{cm}}^3$ is proportional to ${\kappa }_0$.

Figure 3

Schematic of an MRI technique using a planar array of magnetometers. The ambiguity in the size of a spherical magnetization distribution is removed by applying a linear gradient that separates the object into slices in the frequency domain.