  Hybrid turnstiles have proven to generate accurate single-electron currents.
The usual operation consists of applying a periodic modulation to a
capacitively coupled gate electrode and requires a non-zero DC source-drain
bias voltage. Under this operation, a current of the same magnitude and
opposite direction can be generated by flipping the polarity of the bias. Here,
we demonstrate that accurate single-electron currents can be generated under
zero average bias voltage. We achieve this by applying an extra periodic
modulation with twice the frequency of the gate signal and zero DC level to the
source electrode. This creates a time interval, which is otherwise zero,
between the crossings of tunnelling thresholds that enable single-electron
tunnelling. Furthermore, we show that within this operation the current
direction can be reversed by only shifting the phase of the source signal.
