  We developed highly-sensitive image-current detection systems based on
superconducting toroidal coils and ultra-low noise amplifiers for
non-destructive measurements of the axial frequencies (550$\sim$800$\,$kHz) of
single antiprotons stored in a cryogenic multi-Penning-trap system. The
unloaded superconducting tuned circuits show quality factors of up to
500$\,$000, which corresponds to a factor of 10 improvement compared to our
previously used solenoidal designs. Connected to ultra-low noise amplifiers and
the trap system, signal-to-noise-ratios of 30$\,$dB at quality factors of >
20$\,$000 are achieved. In addition, we have developed a superconducting switch
which allows continuous tuning of the detector's quality factor, and to
sensitively tune the particle-detector interaction. This allowed us to improve
frequency resolution at constant averaging time, which is crucial for single
antiproton spin-transition spectroscopy experiments, as well as improved
measurements of the proton-to-antiproton charge-to-mass ratio.
