Demonstration of a quantum controlled-NOT gate in the telecommunications band

We present the first quantum controlled-not (CNOT) gate realized using a fiber-based indistinguishable photon-pair source in the 1.55 mu m telecommunications band. Using this free-space CNOT gate, all four Bell states are produced and fully characterized by performing quantum-state tomography, demonstrating the gate's unambiguous entangling capability and high fidelity. Telecom-band operation makes this CNOT gate particularly suitable for quantum-information-processing tasks that are at the interface of quantum communication and linear optical quantum computing.