Conductance fluctuations in a quantum dot under almost periodic ac pumping

It is shown that the variance of the linear dc-conductance fluctuations in an open quantum dot under a high-frequency ac pumping depends significantly on the spectral content of the ac field. For a sufficiently strong ac field $\gamma {\tau }_{\phi }\ll 1,$ where $1/{\tau }_{\phi }$ is the dephasing rate induced by ac noise and $\gamma$ is the electron escape rate, the dc-conductance fluctuations are much stronger for the harmonic pumping than in the case of the noise ac field of the same intensity. The reduction factor r in a static magnetic field takes the universal value of 2 only for the white-noise pumping. For the strictly harmonic pumping ${A\left(t\right)=A}_0\cos \omega t$ of sufficiently large intensity the variance is almost insensitive to the static magnetic field $r-1=2\sqrt{{\tau }_{\phi }\gamma }\ll 1.$ For the quasiperiodic ac field of the form ${A\left(t\right)=A}_0\left[\cos \right({\omega }_{1}t)+\cos ({\omega }_{2}t\left)\right]$ with ${\omega }_{1,2}\gg \gamma$ and $\gamma {\tau }_{\phi }\ll 1$ we predict effect of enchancement of conductance fluctuations at commensurate frequencies ${\omega }_2/{\omega }_1=P/Q.\mathrm{}$