A spin that damps itself
What this site is and an indulgent introduction
A coherent ensemble of nuclear spins precessing in a coil induces a current; that current makes a field; that field torques the spins that made it, dragging them back toward equilibrium. The signal generates a drag against its own motion. Left alone, the ensemble damps itself.
That’s radiation damping: a system acting on its own coherence, rather than (directly) ceding it to its environment.
Invert the populations and the coupling changes sign. The feedback that damped the ensemble now amplifies it: the spins emit in step, the emission drives more emission, and the system mases instead of decaying. Same mechanism, opposite outcome.
The work these notes are about lives between those two cases. Gain against loss, and what happens where they compete: thresholds, self-sustained oscillation, dynamics not well described by the usual Hermitian formalism. That tension draws in a surprising range of embedding techniques from quantum information science, placing the nuclear Zeeman maser, or RASER, at the intersection of several fields and making it an unusually good pedagogical example. Coming from ZULF and optically-detected NMR, and working in a lab built around parahydrogen-based hyperpolarization, I tend to reach for coherence order as the natural language for coherent processes. The RASER is just one place it pays off.
These are working notes. The early entries will mostly be rough versions of my thesis chapters, though I’ll also indulge some of my other research interests.
— S.A.D.