Landau-Zener interference at bichromatic driving

We investigate experimentally and theoretically the interference at avoided crossings which are repeatedly traversed as a consequence of an applied ac field. Our model system is a charge qubit in a serial double quantum dot connected to two leads. Our focus lies on effects caused by simultaneous driving with two different frequencies. We work out how the commensurability of the driving frequencies affects the symmetry of the interference patterns both in real space and in Fourier space. For commensurable frequencies, the symmetry depends sensitively on the relative phase between the two modes, whereas for incommensurable frequencies the symmetry of monochromatic driving is always recovered.