Experimentally generating and tuning robust entanglement between photonic qubits

We introduce and demonstrate a technique for generating a range of novel multi-photon entangled states. Adjusting a simple experimental parameter allows the preparation of pure states with an arbitrary level of W-class entanglement, from a fully separable state to the maximally robust W state, enabling full control over this entanglement class in our system. Furthermore, the generated states exhibit a highly symmetric entanglement distribution that we show is optimally robust against qubit loss. The ability to prepare entanglement in robust configurations is particularly relevant to many emerging quantum technologies where entanglement is a valuable resource. We achieve a high quality experimental realization for the three-photon case, including a W state fidelity of 0.90 +/- 0.03. In addition, we present a new technique for characterizing quantum states in the laboratory in the form of iterative tomography.