Electron paramagnetic resonance spectroscopy of Er3+:Y2SiO5 using a Josephson bifurcation amplifier: Observation of hyperfine and quadrupole structures

We performed magnetic field and frequency tunable electron paramagnetic resonance spectroscopy of an Er3+-doped Y2SiO5 crystal by observing the change in flux induced on a direct current superconducting quantum interference device (dc SQUID) loop of a tunable Josephson bifurcation amplifer. The observed spectra show multiple transitions which agree well with the simulated energy levels, taking into account the hyperfine and quadrupole interactions of Er-167. The sensing volume is about 0.15 pl, and our inferred measurement sensitivity (limited by external flux noise) is approximately 1.5 x 10(4) electron spins for a 1-s measurement. The sensitivity value is two orders of magnitude better than similar schemes using a dc SQUID switching readout.