  We propose a deterministic source of entangled multiphoton states based on
spontaneous emission from a ladder of a cascaded quantum well structure. The
coupling between the quantum wells enables a many-path evolution with the
emission of photon-number combination states in three modes. The tripartite
multiphoton state can be used for controlling the entanglement between two
multiphoton modes by measuring the third. We further discuss an application as
a qubit-pair source with an error-detection ancilla.
