Experimental demonstration of direct secure communication with 2-qubit states

Single photons are used as carriers for the experimental realization of 2-qubit states. One qubit is encoded in the photon's polarisation, the other one in its time of arrival or its spatial mode. It is therefore possible to prepare and analyse the desired states just with linear optical components. The preparation of a polarisation-time-entangled /spl Phi//sup +/-state is done with an asymmetric Mach-Zehnder-Interferometer (MZI) with polarising beamsplitters (PBS). With a subsequent polarisation rotation all eight basis states can be prepared.