Experimental realization of a two-bit phase damping quantum code

Using nuclear-magnetic-resonance techniques, we experimentally investigated the effects of applying a two-bit phase error detection code to preserve quantum information in nuclear spin systems. Input states were stored with and without coding, and the resulting output states were compared with the originals and with each other. The theoretically expected result, net reduction of distortion and conditional error probabilities to second order, was indeed observed, despite imperfect coding operations which increased the error probabilities by approximately 5%. A systematic study of the deviations from the ideal behavior provided quantitative measures of different sources of error, and good agreement was found with a numerical model. Theoretical questions in quantum error correction in bulk nuclear spin systems including fidelity measures, signal strength, and syndrome measurements are discussed. [S1050-2947(99)02309-4].