  The development of ultrahigh quality factor (Q) microresonators has been
driving such technologies as cavity quantum electrodynamics (QED),
high-precision sensing, optomechanics, and optical frequency comb generation.
Here we report ultrahigh-Q crystalline microresonator fabrication with a Q
exceeding $10^8$, for the first time, achieved solely by computer-controlled
ultraprecision machining. Our fabrication method readily achieved the
dispersion engineering and size control of fabricated devices via programmed
machine motion. Moreover, in contrast to the conventional polishing method, our
machining fabrication approach avoids the need for subsequent careful
polishing, which is generally required to ensure that surface integrity is
maintained, and this enabled us to realize an ultrahigh-Q. We carefully
addressed the cutting condition and crystal anisotropy to overcome the large
surface roughness that has thus far been the primary cause of the low-Q in the
machining process. Our result paves the way for future mass-production with a
view to various photonic applications utilizing ultrahigh-Q crystalline
microresonators.
