  Continuous U(1) gauge symmetry, which guarantees the conservation of the
total excitations in linear bosonic systems, will be broken when it comes to
the strong-coupling regime where the rotation wave approximation (RWA) fails.
Here we develop analytic solutions for multi-mode bosonic systems with XX-type
couplings beyond RWA, and proposed a novel scheme to implement high-fidelity
quantum state transfer (QST) and entanglement preparation (EP) with high speed.
The scheme can be realized with designated coupling strength and pulse duration
with which the excitation number keeps unchanged regardless of the breakdown of
the global U(1) symmetry. In the QST tasks, we consider several typical quantum
states and demonstrate that this method is robust against thermal noise and
imperfections of experimental sequence. In the EP tasks, the scheme is
successfully implemented for the preparation of Bell states and W-type states,
within a shortest preparation time.
