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 q uantum information in nuclear spin systems. Input states were stored with a nd without coding, and the resulting output states were compared with the o riginals and with each other. The theoretically expected result, net reduct ion of distortion and conditional error probabilities to second order, was indeed observed, despite imperfect coding operations which increased the er ror probabilities by approximately 5%. A systematic study of the deviations from the ideal behavior provided quantitative measures of different source s of error, and good agreement was found with a numerical model. Theoretica l questions in quantum error correction in bulk nuclear spin systems includ ing fidelity measures, signal strength, and syndrome measurements are discu ssed. [S1050-2947(99)02309-4].