Optical implementation of two programmable quantum measurement devices

We present an optical realization of an adjustable discriminator of non-orthogonal quantum states and a programmable phase-covariant quantum multimeter. Both devices utilize polarization states of photon pairs generated by spontaneous parametric down-conversion. The experimental realization is based on the fact that two Bell states can be distinguished solely by means of linear optics. The first device discriminates unambiguously between two non-orthogonal polarization states of the data qubit. The selection of the states that should be discriminated is controlled by the quantum state of the program qubit. The second device can perform any von Neumann measurement on a single qubit represented by a polarization state from the equator of the Bloch sphere. Also in this case the measurement basis is selected by the state of the program qubit. (author)