  Optical qubits uniquely combine information transfer in optical fibers with a
good processing capability and are therefore attractive tools for quantum
technologies. A large challenge, however, is to overcome the low efficiency of
two-qubit logic gates. The experimentally achieved efficiency in an optical
controlled NOT (CNOT) gate reached approximately 11% in 2003 and has seen no
increase since. Here we report on a new platform that was designed to surpass
this long-standing record. The new scheme avoids inherently probabilistic
protocols and, instead, combines aspects of two established quantum nonlinear
systems: atom-cavity systems and Rydberg electromagnetically induced
transparency. We demonstrate a CNOT gate between two optical photons with an
average efficiency of 41.7(5)% at a postselected process fidelity of 81(2)%.
Moreover, we extend the scheme to a CNOT gate with multiple target qubits and
produce entangled states of presently up to five photons. All these
achievements are promising and have the potential to advance optical quantum
information processing in which almost all advanced protocols would profit from
high-efficiency logic gates.
