Spectroscopy of double quantum dot two-spin states by tuning the interdot barrier

Transport spectroscopy of two-spin states in a double quantum dot can be performed by an ac electric field which tunes the energy detuning. However, a problem arises when the transition rate between the states is small and, consequently, the ac-induced current is suppressed. Here we show that if the ac field tunes the interdot tunnel barrier, then for large detuning the transition rate increases drastically, resulting in high current. Multiphoton resonances are enhanced by orders of magnitude. Our study demonstrates an efficient way for fast two-spin transitions.