Universal unitary gate for single-photon two-qubit states - art. no. 032303

Upon entangling a spatial binary alternative of a photon with its polarizat ion, one can use single photons to study arbitrary 2-qubit states. Sending the photon through a Mach-Zehnder interferometer, equipped with sets of wav e plates that change the polarization, amounts to performing a unitary tran sformation on the 2-qubit state. We show that any desired unitary gate can be realized by a judicious choice of the parameters of the setup and discus s a number of applications. They include the diagnosis of an unknown 2-qubi t state, an optical Grover search, and the realization of a thought experim ent invented by Vaidman. Aharonov. and Albert.