Scalable Spin Squeezing for Quantum-Enhanced Magnetometry with   Bose-Einstein Condensates
  A major challenge in quantum metrology is the generation of entangled states with macroscopic atom number. Here, we demonstrate experimentally that atomic squeezing generated via non-linear dynamics in Bose Einstein condensates, combined with suitable trap geometries, allows scaling to large ensemble sizes. We achieve a suppression of fluctuations by 5.3(5) dB for 12300 particles, which implies that similar squeezing can be achieved for more than 10$^7$ atoms. With this resource, we demonstrate quantum-enhanced magnetometry by swapping the squeezed state to magnetically sensitive hyperfine levels that have negligible nonlinearity. We find a quantum-enhanced single-shot sensitivity of 310(47) pT for static magnetic fields in a probe volume as small as 90 $\mu$m$^3$. 
W. Muessel, H. Strobel, D. Linnemann, D. B. Hume, M. K. Oberthaler
quant-ph cond-mat.quant-gas