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Future applications of quantum physics for quantum simulations, computation, communication and metrology will require an extremely high degree of control of the preparation, manipulation and detection of strongly correlated states of quantum many-body systems. Cold atoms offer an unprecedented playground for the realization of these goals. One such highly correlated state is the singlet state, i.e., a state with total spin zero and no spin fluctuations. It appears as ground state of many fundamental spin models and is invariant under the unitary transformation which describes the effect of an external magnetic field on the spins. Possible applications of such state include encoding quantum information in a decoherence free subspace, sending information independently of the reference frame, or metrological applications in which insensitivity to external homogenous magnetic fields is needed.

One of the project in our lab is generating a macroscopic singlet state in a cold atomic ensemble. The experiment is based on a recent proposal to generate these states by applying a quantum non-demolition (QND) measurement and feedback scheme to an unpolarized ensemble. It has been experimentally demonstrated and theoretically investigated that QND measurement is a useful tool for reducing spin fluctuations i.e. spin squeezing.