Phase-locked amplification enhanced by spin squeezing

Abstract: Quantum lock-in amplification raises the detection sensitivity of magnetic fields to unprecedented level by phase-locked pumping the Zeeman levels of a single trapped atom. To further enhance this sensitivity, quadrature squeezing could be introduced to overcome the quantum uncertainty limit. We propose a detection scheme using an atomic ensemble whose collected spin is pumped by two lasers for simultaneous squeezing and phase locking. We derive the optimal $\pi/2$-pulse and $\pi$-pulse schemes that accomplishes this concurrent action and prove that the resulting phase sensitivity is enhanced while the usable detection window for phase locking is widened.