We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of 2 fT/Hz(1/2) at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about 0.01 fT/Hz(1/2) can be achieved in a practical system with a measurement volume of 200 cm3.

We describe a Cs atomic magnetometer operating in the spin-exchange relaxation-free (SERF) regime. With a vapor cell temperature of 103 • C we achieve intrinsic magnetic resonance widths ∆B = 17 µG corresponding to an electron spin-relaxation rate of 300 s −1 when the spin-exchange rate is ΓSE = 14000 s −1 . We also observe an interesting narrowing effect due to diffusion. Signalto-noise measurements yield a sensitivity of about 400 pG/ √ Hz. Based on photon shot noise, we project a sensitivity of 40 pG/ √ Hz. A theoretical optimization of the magnetometer indicates sensitivities on the order of 2 pG/ √ Hz should be achievable in a 1 cm 3 volume. Because Cs has a higher saturated vapor pressure than other alkali metals, SERF magnetometers using Cs atoms are particularly attractive in applications requiring lower temperatures.

We demonstrate detection of NMR signals using a non-cryogenic atomic magnetometer and describe several novel applications of this technique. A water free induction decay (FID) signal in a 0.5 µT field is detected using a spin-exchange-relaxation-free K magnetometer and the possibility of using a multi-channel magnetometer for 3-D MRI requiring only a single FID signal is described. We also demonstrate detection of less than 10 13 129 Xe atoms whose NMR signal is enhanced by a factor of 540 due to Fermi-contact interaction with K atoms. This technique allows detection of less than 10 9 129 Xe spins in a flowing system suitable for remote NMR applications.PACS numbers: 33.25.+k,83.85.Fg, Nuclear magnetic resonance is a powerful technique widely used in both basic research and medical applications. Traditionally, NMR signals from thermal nuclear polarization are detected with an RF pick-up coil. A high magnetic field, usually produced by a superconducting magnet, increases the strength of the signal approximately as B 2 and improves the ability to resolve NMR chemical shifts. In applications that do not require chemical shift information it is possible to avoid using a large magnetic field by utilizing a magnetometer instead of an inductive pick-up coil to detect the NMR signal, making the signal strength proportional only to the first power of B. Detection of NMR signals in fields as low as a few µT has been demonstrated using SQUID magnetometers [1,2]. It is even possible to completely eliminate the dependence of the signal strength on the magnetic field by utilizing hyperpolarized nuclei, such as 129 Xe polarized by spin-exchange optical pumping [3] or protons polarized by Spin Polarization Induced Nuclear Overhauser Effect [4]. However, NMR detection with SQUID magnetometers still requires a cryogenic system and precludes many applications of NMR and MRI requiring portable, maintenance-free systems.In this Letter we demonstrate NMR detection using an atomic magnetometer and describe several novel applications based on unique properties of atomic magnetometers. Recent advances in atomic magnetometry [5], in particular demonstration of a spin-exchangerelaxation-free (SERF) magnetometer [6] have allowed alkali-metal magnetometers to exceed the sensitivity of low-temperature SQUID detectors [7]. In addition to high sensitivity, atomic magnetometers also allow lowcost multi-channel measurements with a high spatial resolution and do not require cryogenic cooling. Here we demonstrate first detection of NMR free induction decay (FID) signals from a thermally polarized water sample with an atomic magnetometer. Previous measurements using atomic magnetometers have only detected DC magnetization of hyperpolarized gases [8,9,10]. We also describe a novel MRI method that allows reconstruction of a 3-D image from a single FID signal in the presence of a constant magnetic field gradient by relying on multichannel magnetic field measurements. In a separate experiment, we investigate a unique method for enhancing NMR sensitivit...

One of the challenges in functional brain imaging is integration of complementary imaging modalities, such as magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI). MEG, which uses highly sensitive superconducting quantum interference devices (SQUIDs) to directly measure magnetic fields of neuronal currents, cannot be combined with conventional high-field MRI in a single instrument. Indirect matching of MEG and MRI data leads to significant co-registration errors. A recently proposed imaging method--SQUID-based microtesla MRI--can be naturally combined with MEG in the same system to directly provide structural maps for MEG-localized sources. It enables easy and accurate integration of MEG and MRI/fMRI, because microtesla MR images can be precisely matched to structural images provided by high-field MRI and other techniques. Here we report the first images of the human brain by microtesla MRI, together with auditory MEG (functional) data, recorded using the same seven-channel SQUID system during the same imaging session. The images were acquired at 46 microT measurement field with pre-polarization at 30 mT. We also estimated transverse relaxation times for different tissues at microtesla fields. Our results demonstrate feasibility and potential of human brain imaging by microtesla MRI. They also show that two new types of imaging equipment--low-cost systems for anatomical MRI of the human brain at microtesla fields, and more advanced instruments for combined functional (MEG) and structural (microtesla MRI) brain imaging--are practical.

Nuclear magnetic resonance (NMR) in liquids and solids is primarily detected by recording the net dipolar magnetic field outside the spin-polarized sample. But the recorded bulk magnetic field itself provides only limited spatial or structural information about the sample. Most NMR applications rely therefore on more elaborate techniques such as magnetic field gradient encoding or spin correlation spectroscopy, which enable spatially resolved imaging and molecular structure analysis, respectively. Here we demonstrate a fundamentally different and intrinsically information-richer modality of detecting NMR, based on the rotation of the polarization of a laser beam by the nuclear spins in a liquid sample. Optical NMR detection has in fact a long history in atomic vapours with narrow resonance lines, but has so far only been applied to highly specialized condensed matter systems such as quantum dots. It has been predicted that laser illumination can shift NMR frequencies and thus aid detection, but the effect is very small and has never been observed. In contrast, our measurements on water and liquid 129Xe show that the complementary effect-the rotation of light polarization by nuclear spins-is readily measurable, and that it is enhanced dramatically in samples containing heavy nuclei. This approach to optical NMR detection should allow correlated optical and NMR spectroscopy on complex molecules, and continuous two-dimensional imaging of nuclear magnetization with spatial resolution limited only by light diffraction.