Room-temperature ultrasensitive mass spectrometer via dynamical decoupling

We propose an ultrasensitive 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 the time domain. The time-comb structure enables high-precision measurements of oscillator frequency, which can be used as an ultrasensitive mass spectrometer. We show that, in the ideal case, the sensitivity $\eta$ of the proposed mass spectrometer has better performance at higher temperature and scales with the temperature $T$ as $\eta \sim T^{-1/2}$. While taking into account qubit and oscillator decay, the optimal sensitivity reaches a universal value independent of environmental temperature $T$. The measurement sensitivity $\eta$ also shows an improved dependence on the control-pulse number $N$ as $\eta \sim N^{-3/2}$, in comparison with the $N^{-1/2}$ scaling in previous magnetometry studies. With the present technology on solid-state spin qubit and high-quality optomechanical system, our proposal is feasible to realize an ultrasensitive room-temperature mass spectrometer.

Figure 1

Schematic of the proposed mass spectrometer. A nanodiamond is trapped in a harmonic potential by counterpropagating laser beams. The nanodiamond (the gray circle) contains a NV center, which serves as a qubit. With a gradient magnetic field, the center-of-mass motion of the nanodiamond couples to the NV-center spin. Under DD control, the qubit coherence exhibits a time-comb structure, which can be used to measure the tiny change of the oscillation frequency (thus the mass change) of the nanodiamond.

Figure 2

(a) The time-comb structure of qubit coherence under 100-pulse CPMG control. For ${\omega }_{0}t\gg 1$, the comb period is synchronized with the oscillator period $T_0=2\pi /{\omega }_0$. (b) Closeup of the coherence peaks. A missing peak at ${\omega }_{0}t=N\pi$ is indicated by the blue dashed line. The peak width is decreasing when it gets close to the missing one. (c) Closeup of the narrowest coherence peak, which is centered at $t_{q^{*}}$ with width ${\Delta }_{q^{*}}$ (see text). The parameters used in this figure are oscillator frequency ${\omega }_0/\left(2\pi \right)=100$ kHz, coupling strength $\lambda =0.001{\omega }_0$, temperature $T=10$ K, and 100-pulse CPMG control.

Figure 3

Mass sensitivity ${\eta }_M$ as functions of DD control pulse number $N$. Solid curves are the sensitivity at room temperature $T=300$ K, while dashed curves are the sensitivity at low temperature $T=1$ K. The red curves are the ideal sensitivity according to Eq. (6), which scale as $\sim N^{-3/2}$. The curves in green, blue, and orange are the sensitivity taking into account the qubit $T_1$ decay (with $T_1=7$ ms), qubit $T_2$ decay (with $T_2=100\mu \text{s})$, and oscillator finite $Q$ factor (with $Q={10}^9)$, in turn. The thick black curves are the sensitivity with all the decay mechanisms included. The horizontal dotted line indicates the temperature-independent optimal sensitivity. The other parameters $({\omega }_0$ and $\lambda )$ used in this figure are the same as those in Fig. 2.