  In this paper, we continue our investigation on controlling the state of a
quantum harmonic oscillator, by coupling it to a reservoir composed of a
sequence of qubits. Specifically, we show that sending qubits separable from
each other but initialised at different states in pairs can stabilise the
oscillator at squeezed states. However, only if entanglement is allowed in the
reservoir qubit can we stabilise the oscillator at a wider set of squeezed
states. This thus provides a proof for the necessity of involving entanglement
in the reservoir qubits input to the oscillator, as regard to the stabilisation
of quantum states in the proposed system setting. On the other hand, this
system setup can be in turn used to estimate the coupling strength between the
oscillator and reservoir qubits. We further demonstrate that entanglement in
the reservoir input qubits contributes to the corresponding quantum Fisher
information. From this point of view, entanglement is proved to play an
indispensable role in the improvement of estimation precision in quantum
metrology.
