  We propose an ultra-sensitive mass spectrometer based on a coupled
quantum-bit-oscillator system. Under dynamical decoupling control of the
quantum bit (qubit), the qubit coherence exhibits a comb structure in time
domain. The time-comb structure enables high precision measurement of
oscillator frequency, which can be used as an ultra-sensitive mass
spectrometer. Surprisingly, in ideal case, the sensitivity of the proposed mass
spectrometer, which scales with the temperature $T$ as $T^{-1/2}$, has better
performance in higher temperature. While taking into account qubit and
oscillator decay, we show that the optimal sensitivity is independent on
environmental temperature $T$. With present technology on solid state spin
qubit and high-quality optomechanical system, our proposal is feasible to
realize an ultra-sensitive mass spectrometer in room temperature.
