Generation of the four-photon W state and other multiphoton entangled states using parametric down-conversion
Hideharu Mikami, Yongmin Li, Takayoshi Kobayashi · Physical Review A · 2004
We propose two interesting methods of generating the four-photon $W$ state. These methods use parametric down-conversion processes, linear optical elements, and commercial photon detectors, which are readily feasible under current technology. They can also be used to generate the three-photon $W$ state, the three-photon Greenberger-Horne-Zeilinger state, and the three-photon maximally entangled photon-number state (a typical photon-number entanglement state) by simply changing some experimental components or their parameters. Moreover, assuming we have photon number-resolving detectors, these methods can develop into methods that generate a general $n$-photon $W$ state. They are expected to become powerful tools for experimental investigations of multipartite entanglement and its applications to quantum information processing.