Towards hyperentangled states of two photons and six qubits

We discuss two possible ways to increase the dimension of a 4-qubit hyperentangled state of two photons generated through the spontaneous parametric down conversion (SPDC) process. The two techniques are respectively based on the continuum of k-modes generated by a type I phase matched crystal and on the adoption of the time-energy entanglement as a further degree of freedom of the photons. The realization of an entangled state of two photons encoded in six (or even more) qubits, which can be transformed in a cluster state by suitable C-Phase gates, may represent a significant step towards the realization of efficient quantum computation based on the one-way model. © 2009 World Scientific Publishing Company