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We demonstrate the creation of entangled, spin-squeezed states using a collective, or joint, measurement and real-time feedback. The pseudospin state of an ensemble of N=5×10^4 laser-cooled ^87Rb atoms is deterministically driven to a specified population state with angular resolution that is a factor of 5. 5 (8) 7. 4 (6) dB in variance below the standard quantum limit for unentangled atoms-comparable to the best enhancements using only unitary evolution. Without feedback, conditioning on the outcome of the joint premeasurement, we directly observe up to 59 (8) times 17. 7 (6) dB improvement in quantum phase variance relative to the standard quantum limit for N=4×10^5 atoms. This is one of the largest reported entanglement enhancements to date in any system.
Cox et al. (Fri,) studied this question.