Correlated and collective stochastic dephasing of qubit entanglement

Decoherence of qubit entanglement caused by correlated and collective stochastic dephasing is investigated. It is shown that in the correlated dephasing, the correlation between the stochastic processes which affect individual qubits makes greater or smaller the decoherence of qubit entanglement, depending on properties of two-qubit states. On the other hand, in the collective dephasing, the collectiveness of the stochastic process cannot suppress the decoherence.