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In this paper we prove an infinite dimensional KAM theorem, in which theassumptions on the derivatives of perturbation in \cite{GT} are weakened frompolynomial decay to logarithmic decay. As a consequence, we apply it to 1dquantum harmonic oscillators and prove the reducibility of a linear harmonicoscillator, $T=- \frac{d^2}{dx^2}+x^2$, on $L^2(\R)$ perturbed by aquasi-periodic in time potential $V(x,\omega t; \omega)$ with logarithmicdecay. This entails the pure-point nature of the spectrum of the Floquetoperator $K$, where K:=-{\rm i}\sum_{k=1}^n\omega_k\frac{\partial}{\partial\theta_k}- \frac{d^2}{dx^2}+x^2+\varepsilon V(x,\theta;\omega), is defined on$L^2(\R) \otimes L^2(\T^n)$ and the potential $V(x,\theta;\omega)$ haslogarithmic decay as well as its gradient in $\omega$.

1d Quantum Harmonic Oscillator Perturbed by a Potential with Logarithmic Decay