Dynamical detection of Majorana fermions in current-biased nanowires

We analyze the current-biased Shapiro experiment in a Josephson junction formed by two one-dimensional nanowires featuring Majorana fermions. Ideally, these junctions are predicted to have an unconventional $4\ensuremath{\pi}$-periodic Josephson effect and thus only Shapiro steps at even multiples of the driving frequency. Taking additionally into account the overlap between the Majorana fermions, due to the finite length of the wire, renders the Josephson junction conventional for any dc experiments. We show that probing the current-phase relation in a current-biased setup dynamically decouples the Majorana fermions. We find that, besides the even integer Shapiro steps, there are additional steps at odd and fractional values. However, different from the voltage-biased case, the even steps dominate for a wide range of parameters even in the case of multiple modes, thus giving a clear experimental signature of the presence of Majorana fermions.