Gardiner's phonon for Bose-Einstein Condensation: A physical realization of the $q$-deformed Boson

The Gardiner's phonon presented for a particle-number conserving approximation method to describe the dynamics of Bose-Einstein Condesation (BEC) (C.W. Gardiner, Phys. Rev. A 56, 1414 (1997)) is shown to be a physical realization of the $q$-deformed boson, which was abstractly developed in quantum group theory. On this observation, the coherent output of BEC atoms driven by a radio frequency (r.f) field is analyzed in the viewpoint of a $q$-deformed Fock space. It is illustrated that the $q$-deformation of bosonic commutation relation corresponds to the non-ideal BEC with the finite particle number $N$ of condensated atoms. Up to order 1/N, the coherent output state of the untrapped atoms minimizes the uncertainty relation like a coherent state does in the ideal case of BEC that $N$ approaches infinity or $q=1-2/N$ approaches one.