Quantum-enhanced sensing by echoing spin-nematic squeezing in atomic Bose–Einstein condensate

Quantum entanglement can provide enhanced precision beyond standard quantum limit (SQL), the highest precision achievable with classical means. It remains challenging, however, to observe large enhancement limited by the experimental abilities to prepare, maintain, manipulate and detect entanglement. Here, we present nonlinear interferometry protocols based on echoing spin-nematic squeezing to achieve record high enhancement factors in atomic Bose-Einstein condensate. The echo is realized by a state-flip of the spin-nematic squeezed vacuum, which serves as the probe state and is refocused back to the vicinity of the unsqueezed initial state while carrying out near noiseless amplification of a signal encoded. A sensitivity of $21.6\pm0.5$ decibels (dB) for a small-angle Rabi rotation beyond the two-mode SQL of 26400 atoms as well as $16.6\pm1.3$ dB for phase sensing in a Ramsey interferometer are observed. The absolute phase sensitivity for the latter extrapolates to $103~\rm{pT/\sqrt{Hz}}$ at a probe volume of $18~\mu\rm{m}^3$ for near-resonant microwave field sensing. Our work highlights the excellent many-body coherence of spin-nematic squeezing and suggests its possible quantum metrological applications in atomic magnetometer, atomic optical clock, and fundamental testing of Lorentz symmetry violation, etc.