  We have successfully used a fast electronic feed forward to increase the
success probability of a linear optical implementation of a programmable phase
gate from 25% to its theoretical limit of 50%. The feed forward applies a
conditional unitary operation which changes the incorrect output states of the
data qubit to the correct ones. The gate itself rotates an arbitrary quantum
state of the data qubit around the z-axis of the Bloch sphere with the angle of
rotation being fully determined by the state of the program qubit. The gate
implementation is based on fiber optics components. Qubits are encoded into
spatial modes of single photons. The signal from the feed-forward detector is
led directly to a phase modulator using only a passive voltage divider. We have
verified the increase of the success probability and characterized the gate
operation by means of quantum process tomography. We have demonstrated that the
use of the feed forward does not affect either the process fidelity or the
output-state fidelities.
