title: Deterministic Squeezed States with Collective Measurements and Feedback.
abstract: 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 ^{87}Rb 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.
authors.affiliation: JILA, NIST, and University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Cox
authors.affiliation: JILA, NIST, and University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: Graham P
authors.initials: GP
authors.lastname: Greve
authors.affiliation: JILA, NIST, and University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: Joshua M
authors.initials: JM
authors.lastname: Weiner
authors.affiliation: JILA, NIST, and University of Colorado, 440 UCB, Boulder, Colorado 80309, USA.
authors.forename: James K
authors.initials: JK
authors.lastname: Thompson