Taking Coherence to Extremes

In most nuclear magnetic resonance (NMR) experiments, a uniform magnetic field is applied to the sample, and radio frequency pulses are used to manipulate particular nuclear spins. Under ex situ conditions, such as deep in a bore hole, it is difficult to apply high magnetic fields or to do so uniformly, and, thus, approaches in which signals are generated by suddenly reversing the applied magnetic field prove useful. Unfortunately, background fields (such as Earth’s field) are difficult to shield, and the resulting signals last only a few milliseconds. Brill et al. (p. 369) show that when two nearly orthogonal fields are reversed in such a way so as to compensate for the background field, spin echoes form that persist for more than 600 milliseconds, which allows accurate measurement of spin-relaxation times.