Ryotaro Inoue, Shin-Ichi-Ro Tanaka, Ryo Namiki, Takahiro Sagawa, Yoshiro Takahashi
We demonstrate an unconditional spin squeezing in a collective spin system via feedback control based on quantum non-demolition measurement (QNDM). We perform shot-noise limited collective spin measurements on an ensemble of $3.7\times 10^5$ laser-cooled ${}^{171}\text{Yb}$ atoms in their spin-1/2 ground states. %The quantum noise suppression associated with measurement-induced inter-particle correlation or quantum entanglement is the key to the achievement of the spin squeezing. Correlation between two sequential QNDMs indicates generation of conditional spin squeezed states with $-0.80^{+0.11}_{-0.12}\,\mathrm{dB}$ of quantum noise reduction. Our feedback control successfully converts the conditional squeezing into the unconditional squeezing without significant loss of the squeezing level.
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http://arxiv.org/abs/1301.1016
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