Operational multipartite entanglement classes for symmetric photonic qubit states

We present experimental schemes that allow us to study the entanglement classes of all symmetric states in multiqubit photonic systems. We compare the efficiency of the proposed schemes and highlight the relation between the entanglement properties of symmetric Dicke states and a recently proposed entanglement scheme for atoms. In analogy to the latter, we obtain a one-to-one correspondence between well-defined sets of experimental parameters and multiqubit entanglement classes inside the symmetric subspace of the photonic system.

Figure 1

Proposed experimental setups for the observation of arbitrary symmetric states in an $N$-qubit photonic system. A one-to-one correspondence exists between experimental parameter configurations and entanglement SLOCC classes of the observed photonic states. (a) Photons from single photon sources are combined with BSs and are distributed symmetrically through the $N$ modes of an output multiport. (b) The photons are produced from noncollinear spontaneous parametric down-conversion (SPDC) processes and the desired state is prepared by projection of the noncollinear SPDC output photons occupying modes $b_1,\dots ,b_N$. (c) The symmetric photonic states are obtained by use of a collinear SPDC and subsequent projective measurements in half of the output modes (see text for detailed explanations of the schemes).