Greenberger-Horne-Zeilinger state
In physics, in the area of quantum information theory, a Greenberger–Horne–Zeilinger state is a certain type of entangled quantum state which involves at least three subsystems (particles). It was first studied by Daniel Greenberger, Michael Horne and Anton Zeilinger in 1989. They have noticed the extremely non-classical properties of the state.
The GHZ state is an entangled quantum state of M > 2 subsystems. In the case of each of the subsystems being two-dimensional, that is for qubits, it reads
In simple words it is a quantum superposition of all subsystems being in state 0 with all of them being in state 1 (states 0 and 1 of a single subsystem are fully distinguishable).
The simplest one is the 3-qubit GHZ state:
This state is non-biseparable and is the representative of one of the two non-biseparable classes of three-qubit states (the other being the W state) which cannot be transformed (not even probabilistically) into each other by local quantum operations. Thus and represent two very different kinds of tripartite entanglement. The W state is, in a certain sense "less entangled" than the GHZ state; however, that entanglement is, in a sense, more robust against single-particle measurements, in that, for an N-qubit W state, an entangled (N-1) qubit state remains after a single particle measurement. By contrast, certain measurements on the GHZ state collapse it into a mixture or a pure state.
...
Wikipedia