This paper deals with the derivation of a sharp estimate on the difference of traces of the one-parameter Schrödinger semigroup associated to the quantum isotropic harmonic oscillator. Denoting by H∞,κ the self-adjoint realization ine −tH L,κ , t > 0 has for L sufficiently large a Gaussian decay in L. Furthermore, the estimate that we derive is sharp in the two following senses: its behavior when t ↓ 0 is similar to the one given by Tr L 2 (R d ) e −tH∞,κ = (2 sinh( κ 2 t)) −d and the exponential decay in t arising from Tr L 2 (R d ) e −tH∞,κ when t ↑ ∞ is preserved. For illustrative purposes, we give a simple application within the framework of quantum statistical mechanics.MSC-2010 number: 35J10, 47D08, 81Q10, 81Q15, 82B10.