Inductive detection using a tuned circuit turned out to be a very successful technique at high magnetic fields of several tesla. Recent instrumental and methodical advances that were major focal points in our research like ex-situ NMR or remote detection facilitate new spectroscopy and imaging experiments at lower magnetic fields in the sub-tesla range. These techniques demand more sensitive low-field detection methods, which has been done autonomous as well as in collaboration with the Groups of Prof. John Clarke and Prof. Dimitry Budker, both in the Physics Department of UC California.

The atomic magnetometer directly measures magnetic flux density via optical rotation of a laser beam by alkali metal vapor. In the presence of an external magnetic field, a polarized laser beam with a near-resonant wavelength to the atoms’ electronic transition illuminates the alkali vapor, populating certain Zeeman levels while depleting some other levels, depending on the polarization of the light. The induced atomic polarization undergoes Larmor precession associated with the Zeeman shifts. As a result, the polarization of the probe laser beam will be rotated as the beam goes through the vapor. Magneto-optical resonance occurs when the modulation frequency of the laser equals to twice the Larmor precession frequency. The magnitude of the external magnetic field can thus be deduced from its corresponding modulation frequency when resonance is observed.
By forming a magnetic gradiometer with two atomic magnetometers, the common-mode noise can be eliminated. For applications in nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI), a long solenoid that pierces the magnetic shields is employed to provide a ~0.5 G leading field for the nuclear spins in the sample. Our apparatus is suited for remote detection of NMR and MRI, as demonstrated with time-resolved flow of water.

Conventionally, NMR signals are detected with inductive pickup coils in large magnetic fields (>6T) where chemical shifts are mainly used to characterize and analyze molecular structures. The use of laser magnetometers for detection of NMR in low magnetic fields is particularly attractive because they do not suffer the loss of sensitivity at low frequencies inherent to pick up coils and they avoid large expensive cryogenically cooled magnets. Instead of using the chemical shift for characterization of chemical structures the scalar “J-coupling” can be used. The J-coupling provides important information about bond and torsion angles spin topology and molecular hybridization. The J-coupling constants can be obtained with an unprecedented accuracy because a resolution of 0.1 Hz is achieved.

Our measurements are performed by prepolarizing a fluid analyte in a 1.8-T field, and then transporting the fluid to a zero-field region inside a magnetic shield. To excite coherences in the nuclear spin system, a pulse of DC magnetic field is applied, rotating the spins of different nuclear species by different angles. The ensuing quantum beats produce a time dependent magnetization which is monitored by the laser magnetometer.

Future work will focus on the development of techniques for two-dimensional NMR in zero field in order to facilitate the study of more complex molecules. Furthermore, novel polarization methodologies such as the use of para-hydrogen will be implemented. Laser magnetometers can be constructed using microfabrication techniques, and multiplexed assays for chemical identification in biology and medicine are envisioned.

The Superconducting QUantum Interference Device (SQUID) is one class of detectors that has been used for low field applications for several years now. It is a very versatile technique for liquid and solid state NMR spectroscopy as well as for magnetic resonance imaging (MRI) applications. Imaging at low field has various advantages like improved T1 contrast, reduced susceptibility artifacts, or a higher skin depth for metalic materials. This allows to image objects inside metal containers (for example a pepper in a Coke can). The feasibility of MRI with remote detection has recently been shown in a model experiment using a circular flow of water, which is being prepolarized in an inhomogeneous electromagnet prior to encoding.