Scalable Spin Squeezing for Quantum-Enhanced Magnetometry with Bose-Einstein Condensates

In this thesis, we experimentally study the generation of metrologically useful spin squeezed states and investigate their scalability to large atom numbers. Two different experimental schemes that generate these entangled spin states are implemented for two internal states of a Bose-Einstein condensate. We investigate both the previously realized one-axis twisting scenario and a new method which relies on the quantum evolution at an unstable fixed point, which we term bifurcation squeezing. The temporal evolution and the atom number dependence of the final states are examined for both schemes, and the optimal conditions for the creation of squeezing are extracted. We find spin squeezing below -7 dB in this two-mode scenario. By use of parallelized squeezing generation of up to 30 independent condensates in a onedimensional lattice potential, we show that the squeezing of the individual condensates can be scaled up to the full ensemble containing more than 12300 atoms. With a differential analysis, which rejects common mode fluctuations, we find a suppression of fluctuations by -5.3(5) dB for the full ensemble. We directly demonstrate the applicability of this quantum resource for enhanced magnetometry, which is implemented via a modified Ramsey sequence. A transfer to a different set of hyperfine states ensures negligible nonlinearity during the interrogation time and strongly enhances the magnetic sensitivity. We find a quantum-enhanced single-shot sensitivity of 310(47) pT with the full ensemble, and apply the technique for an accurate determination of the magnetic field gradient in our setup