  A central theme in quantum information science is to coherently control an
increasing number of quantum particles as well as their internal and external
degrees of freedom (DoFs), meanwhile maintaining a high level of coherence. The
ability to create and verify multiparticle entanglement with individual control
and measurement of each qubit serves as an important benchmark for quantum
technologies. To this end, genuine multipartite entanglement have been reported
up to 14 trapped ions, 10 photons, and 10 superconducting qubits. Here, we
experimentally demonstrate an 18-qubit Greenberger-Horne-Zeilinger (GHZ)
entanglement by simultaneous exploiting three different DoFs of six photons,
including their paths, polarization, and orbital angular momentum (OAM). We
develop high-stability interferometers for reversible quantum logic operations
between the photon's different DoFs with precision and efficiencies close to
unity, enabling simultaneous readout of 262,144 outcome combinations of the
18-qubit state. A state fidelity of 0.708(16) is measured, confirming the
genuine entanglement of all the 18 qubits.
