  Universal logic gates for two quantum bits (qubits) form an essential
ingredient of quantum information processing. However, the photons, one of the
best candidates for qubits, suffer from the lack of strong nonlinear coupling
required for quantum logic operations. Here we show how this drawback can be
overcome by reporting a proof-of-principle experimental demonstration of a
non-destructive controlled-NOT (CNOT) gate for two independent photons using
only linear optical elements in conjunction with single-photon sources and
conditional dynamics. Moreover, we have exploited the CNOT gate to discriminate
all the four Bell-states in a teleportation experiment.
