18-Qubit Entanglement with Six Photons' Three Degrees of Freedom

Full control of multiple degrees of freedom of multiple particles represents a fundamental ability for quantum information processing. We experimentally demonstrate an 18-qubit Greenberger-Horne-Zeilinger entanglement by simultaneous exploiting three different degrees of freedom of six photons, including their paths, polarization, and orbital angular momentum. We develop high-stability interferometers for reversible quantum logic operations between the photons' different degrees of freedom with precision and efficiencies close to unity, enabling simultaneous readout of 2(18) = 262 144 outcome combinations of the 18-qubit state. A state fidelity of 0.708 +/- 0.016 is measured, confirming the genuine entanglement of all 18 qubits.