Quantum nondemolition measurement of spin via the paramagnetic Faraday rotation

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Abstract
We propose a method for quantum nondemolition (QND) measurement of one component of spin by the measurement of polarization rotation of the off-resonant light induced by a paramagnetic Faraday rotation effect. We show that nonclassical states of spins such as the squeezed spin state, the Dicke state, and macroscopic quantum superposed state of spin can be realized corresponding to particular results of QND measurements. Furthermore, it is shown that a nonclassical state of light such as the twin photon state can also be realized with this method.