  Developing nano-mechanical oscillators for ultrasensitive force detection is
of great importance in exploring science. We report our achievement of
ultrasensitive detection of the external force regarding the radio-frequency
electric field by a nano-sensor made of a single trapped $^{40}$Ca$^{+}$ ion
under injection-locking, where squeezing is additionally applied to detection
of the smallest force in the ion trap. The employed ion is confined stably in a
surface electrode trap and works as a phonon laser that is very sensitive to
the external disturbance. The injection-locking drove the ion's oscillation
with phase synchronization, yielding the force detection with sensitivity of
347 $\pm$ 50 yN/$\sqrt{Hz}$. Further with 3 dB squeezing applied on the
oscillation phase variance, we achieved a successful detection of the smallest
force to be 86.5 $\pm$ 70.1 yN.
