Linear optics implementation of symmetric matchgates

An essential requirement for any quantum computing architecture is the existence of a universal set of quantum gates. Examples of such sets are the 2-qubit CNOT gate in combination with arbitrary 1-qubit gates, and the 3-qubit Toffoli gate together with the 1-qubit Hadamard. A third set, which has not been implemented so far, is Valiant´s 2-qubit matchgate G(A,B) which is universal with the SWAP operation. This paper presents how symmetric Valiant gates are decomposed which are sufficient for universal quantum computation, using another universal class of gates - a controlled-Z_thetas gate and single qubit unitaries. Various symmetric matchgates (G(H,H), G(X,X) and G(T,T)) in a two-photon experiment to build quantum logic gates using higher-dimensional Hilbert spaces are implemented. The gate operations are characterized via quantum process tomography.