The Greenberger-Horne-Zeilinger (GHZ) state is a well-known quantum state utilizing genuine multi-particle entanglement. The GHZ state generally consists of more than 2 particles, where each particle has at least two fully distinguishable states, e.g. 0 and 1. Now in the case of the GHZ state, the particles are in a superposition of all being in the 0-state or all being in the 1-state. That is, for each particle, it is completely undefined whether it is 0 or 1. When we measure one particle, it randomly assumes 0 or 1. Then, the other two assume the same state, independent of their separation. This correlation is an example of multi-partite entanglement and it has very interesting implications in fundamental as well as applied sciences.
In quantum communication, for example, photons provide a stable physical system due to their minor interaction with the environment. Thus, it is possible to distribute these entangled photons over distances of up to several hundreds of kilometers without significant loss of entanglement. On the other hand, the weakness of the interaction between photons makes it experimentally hard to create multi-photon entangled states. It took more than 10 years from the theoretical proposal of the GHZ state to the first experimental demonstration in 2000 .
For practical applications, a significant challenge is to produce such multi-particle states with high quality. The present publication “Generation and application of an ultrahigh-fidelity four-photon Greenberger-Horne-Zeilinger state” managed to improve the overlap (fidelity) of the observed four-photon state up to 98% with the theoretically ideal GHZ state. This result clearly demonstrated the increasing understanding of how to engineer an experimental setup to produce very high quality states. Such high quality GHZ states are a possible resource for optical quantum computation. The current publication shows that to engineer an apparatus capable of producing very high quality four-photon GHZ states can very well be made by current technology. Therefore, the presented results in this article provide a significant contribution on how to engineer high-fidelity multi-photon entangled states in practice.
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