The three-particle entangled quantum state named after Greenberger, Horne, and Zeilinger is the GHZ state. The name comes from physicists Daniel Greenberger, Michael Horne, and Anton Zeilinger.
A three-qubit GHZ state is commonly written as (|000⟩ + |111⟩)/√2. Before measurement, the qubits occupy a joint superposition, but measurements in a shared basis reveal striking correlations. The state is not merely a collection of independently correlated particles; its properties depend on the system as a whole.
Greenberger, Horne, and Zeilinger first studied a four-particle version in 1989. A three-particle version was published the following year, with Abner Shimony joining the authors and building on suggestions by N. David Mermin. GHZ experiments provide a direct demonstration of quantum nonlocality without relying on Bell inequalities.
The GHZ state is often compared with the W state, another important three-qubit entangled state. GHZ entanglement can disappear when one qubit is lost, whereas W-state entanglement is more robust. A Bell state, by contrast, normally describes entanglement between two systems.