Eliminating cavity shifts from electron magnetic moment (g-2) measurements

A cylindrical Penning trap is of such high quality that a single electron is observed with good signal-to-noise. The trap is also a good approximation to an ideal cylindrical microwave cavity. This is documented in situ at 4 K with a new probe utilizing trapped electrons. An extraordinary control of the cavity-electron interaction results and opens the way to a new generation of electron magnetic moment measurements, not limited by either cavity frequency shifts or damping widths, and to sideband cooling of an electron motion to very low (mK) temperatures