Physics Colloquium

In a spin system characterized by weak signals and rapid decay times, spin echoes extend the hope of compensation. This is particularly true in Nuclear Quadrupole Resonance NQR, a type of radio-frequency spectroscopy done at zero-field and potentially useful for detection of contraband. The creation of echo trains effectively extends the signal in time by up to 3 orders of magnitude and can reveal the fundamental broadening mechanisms. We explore these broadening mechanisms. In contrast to an echo train, we also show that an echo forms after a single radio-frequency pulse, as is evident in a power-limited regime where the Larmor frequency approaches the linewidth.