Quantum Magnetometers for Rapid Identification of Resonance Frequencies in Explosives, Pharmaceuticals, and Other Substances

George Mason University, Fairfax VA

Investigators

Abstract

Crystalline solids, such as explosives and pharmaceuticals, have intrinsic resonances that provide a unique radio-frequency fingerprint for the material. Simply exciting the sample with a magnetic wave at its resonance frequency, yields a signal. However, weak signal strength and months-long searches for resonances in unstudied materials prevent ready adoption of this inexpensive and simple technique, known as nuclear quadrupole resonance. Quantum magnetometers, with sensitivities better than standard coil detectors, help with the former problem, but will, with this project, be developed to increase the search speed, potentially five orders of magnitude faster than the conventional method. Despite the difficulty of discovering new resonance lines, researchers have pursued the use of quadrupole resonance for substance detection in a few applications of critical importance for security and society; in particular, the detection of explosives and the identification of counterfeit from real medicine at checkpoints. The adoption of quadrupole resonance for checkpoint and standards applications would greatly expand if resonance lines could be quickly identified for new materials. In addition to national security benefits, the project will improve science, technology, engineering and mathematics education within George Mason by forging strong ties between the Electrical Engineering and Physics Departments through revitalized laboratory courses. Furthermore, by creating undergraduate internship projects focused on K-12 educational experiences, fledgling science teachers will be nurtured and seeds of a diverse and competitive technology and engineering workforce will be planted. Atomic magnetometers are fundamentally different sensors than coils. The use of optically aligned atoms as the detection medium and optical read-out of the sensor with a laser gives a better detection sensitivity than coils; noise in the magnetometer is fundamentally limited by quantum fluctuations. Moreover, the operating frequency of the magnetometer can easily be changed to match the frequency of the excitation with a small static magnetic field. In light of these advantages, the excitation frequency can be swept continuously, as opposed to pulsed excitation at a single operating frequency, obviating the need for a point by point search. With pulsed excitation the material must return to an equilibrium condition before another signal is acquired; often long wait times are required between data acquisitions. However by sweeping up through higher frequencies, then back down to the original frequency, the material is automatically returned to equilibrium. In this way, the long wait times that compromise the effectiveness of standard pulse techniques is avoided. As part of the project, the following goals will be met: 1) Magnetometer cells with an active volume of 2 cm3 will be designed and constructed. The resulting magnetometer will have sub-femtoTesla sensitivity. The small volume is critical for the practical implementation of the resonance search with a limited quantity of material. 2) Continuously tuning the magnetometer to a changing frequency will not alter its sensitivity. 3) The sensitivity of the magnetometer can be retained while resonant excitation on the order of a 100 micro Tesla is applied to a sample a couple centimeters away. Coil geometry and common mode rejection schemes using a second magnetometer will be employed. 4) If the above three goals are achieved, the search speed for resonance frequencies can be improved by as much as five orders of magnitude over conventional techniques.