Time-reversal symmetry breaking by ac field: Effect of commensurability in the frequency domain

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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 the dc conductance fluctuations are much stronger for the periodic 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. In general r may deviate from 2 thus signalling on the time-reversal symmetry breaking by the ac field. For the bi-harmonic ac field of the form A(t)=A₀ [cos(ω₁ t)+cos(ω₂ t)] we predict the enchancement of effects of T-symmetry breaking at commensurate frequencies ω₂/ω₁=P/Q. In the high-temperature limit there is also the parity effect: the enchancement is only present if either P or Q is even.