Protecting quantum Fisher information in correlated quantum channels.

Quantum Fisher information (QFI) has potential applications in quantum metrology tasks. We investigate QFI when the consecutive actions of a quantum channel on the sequence of qubits have partial classical correlations. Our results showed that while the decoherence effect is detrimental to QFI, effects of such classical correlations on QFI are channel-dependent. For the Bell-type probe states, the classical correlations on consecutive actions of the depolarizing and phase flip channels can be harnessed to improve QFI, while the classical correlations in the bit flip and bit-phase flip channels induce a slight decrease of QFI. For a more general parameterization form of the probe states, the advantage of using initial correlated system on improving QFI can also be remained in a wide regime of the correlated quantum channels.