Protected subspace Ramsey spectroscopy

We study a modified Ramsey spectroscopy technique employing slowly decaying states for quantum metrology applications using dense ensembles. While closely positioned atoms exhibit superradiant collective decay and dipole-dipole induced frequency shifts, recent results \[Ostermann, Ritsch and Genes, Phys. Rev. Lett. 111, 123601 (2013)\] suggest the possibility to suppress such detrimental effects and achieve an even better scaling of the frequency sensitivity with interrogation time than for noninteracting particles. Here we present an in-depth analysis of this 'protected subspace Ramsey technique' using improved analytical modeling and numerical simulations including larger 3D samples. Surprisingly we find that using sub-radiant states of N particles to encode the atomic coherence yields a scaling of the optimal sensitivity better than 1/sqrt{N}. Applied to ultracold atoms in 3D optical lattices we predict a precision beyond the single atom linewidth.