We introduce a new kind of continuous-variable-type entangled pure states, called single-mode excited entangled coherent states (SMEECSs), which are obtained through actions of a creation operator of a single-mode optical field on the ECSs. We study the mathematical properties and entanglement characteristics of the SMEECSs, and investigate the influence of photon excitations on quantum entanglement. It is shown that the SMEECSs form a type of cyclic representation of the Heisenberg–Weyl algebra. It is found that the photon excitations seriously affect the entanglement character of the SMEECSs. We also show how such states can be produced by using cavity QED and quantum measurements.
We study Greenberger-Horne-Zeilinger--type (GHZ-type) and
W-type three-mode entangled coherent states. Both types of entangled coherent states violate Mermin's version of the Bell inequality with threshold photon detection (i.e., without photon counting). Such an experiment can be performed using linear optics elements and threshold detectors with significant Bell violations for GHZ-type entangled coherent states. However, to demonstrate Bell-type inequality violations for
W-type entangled coherent states, additional nonlinear interactions are needed. We also propose an optical scheme to generate
W-type entangled coherent states in free-traveling optical fields. The required resources for the generation are a single-photon source, a coherent state source, beam splitters, phase shifters, photodetectors, and Kerr nonlinearities. Our scheme does not necessarily require strong Kerr nonlinear interactions; i.e., weak nonlinearities can be used for the generation of the
W-type entangled coherent states. Furthermore, it is also robust against inefficiencies of the single-photon source and the photon detectors.