Force sensors with precision beyond the standard quantum limit
We propose force sensing protocols using linear ion chain which can operate beyond the quantum standard limit. We show that oscillating forces that are off-resonance with the motional trap frequency can be detected very efficiently by using quantum probes represented by various spin-boson models. We demonstrate that the temporal evolution of a quantum probe described by the Dicke model can be mapped on the nonlinear Ramsey interferometry which allows to detect far-detuned forces simply by measuring the collective spin populations. Moreover, we show that the measurement uncertainty can reach the Heisenberg limit by using initial spin correlated states, instead of motional entangled states. An important advantage of the sensing technique is its natural robustness against the thermally induced dephasing, which extends the coherence time of the measurement protocol. Furthermore, we introduce sensing scheme that utilize the strong spin-phonon coupling to improve the force estimation. We show that for quantum probe represented by quantum Rabi model the force sensitivity can overcome those using simple harmonic oscillator as a force sensor.