  We experimentally demonstrate a programmable quantum gate that applies a sign
flip operation to data qubit in an arbitrary basis fully specified by the
quantum state of a two-qubit program register. Our linear-optical
implementation is inspired by teleportation-based schemes for quantum computing
and relies on multiphoton interference and coincidence detection. We confirm
the programmability of the gate and its high-fidelity performance by carrying
out full quantum process tomography of the resulting operation on data qubit
for several different programs.
