We study the possibility of creating many-particle macroscopic quantum superposition ͑Schrödinger cat͒like states by using a Feshbach resonance to reverse the sign of the scattering length of a Bose-Einstein condensate trapped in a double-well potential. To address the issue of the experimental verification of coherence in the catlike state, we study the revival of the initial condensate state in the presence of environmentally induced decoherence. As a source of decoherence, we consider the interaction between the atoms and the electromagnetic vacuum, due to the polarization induced by an incident laser field. We find that the resulting decoherence is directly related to the rate at which spontaneously scattered photons carry away sufficient information to distinguish between the two atom distributions which make-up the cat state. We show that for a "perfect" cat state, a single scattered photon will bring about a collapse of the superposition, while a less-than-perfect catlike state can survive multiple scatterings before the collapse occurs. In addition, we study the dephasing effect of atom-atom collisions on the catlike states.
Quantum Fisher information of a parameter characterizes the sensitivity of the state with respect to changes of the parameter. In this article, we study the quantum Fisher information of a state with respect to SU(2) rotations under three decoherence channels: the amplitude-damping, phase-damping, and depolarizing channels. The initial state is chosen to be a Greenberger-Horne-Zeilinger state of which the phase sensitivity can achieve the Heisenberg limit. By using the Kraus operator representation, the quantum Fisher information is obtained analytically. We observe the decay and sudden change of the quantum Fisher information in all three channels.
We present a scheme to enhance precision of parameter estimation (PPE) in noise systems via employing dynamical decoupling pulses. The exact analytical expression for the estimation precision of unknown parameter is obtained by using the transfer matrix and time-dependent Kraus operators. We show that PPE in noise systems can be preserved in the Heisenberg limit by controlling of dynamical decoupling pulses. It is found that larger number of pulses and longer reservoir correlation time can remarkably protect the PPE.
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