The study of communication in the presence of noise is central in information theory.  A classic result is Shannon's formula for the capacity of a noisy communication channel.  This theoretical bound on the performance of error correcting codes quantifies the amount of information that can be protected, with vanishing error in the limit of many transmissions.  Protecting quantum mechanical systems from noise can be viewed as an extension of Shannon's problem, where the quantum capacity of a noisy quantum process is defined similarly.  However, no general formula analogous to Shannon's is known, so even apparently simple questions, such as whether a given type of noise allows the transmission of any quantum information at all, are currently unresolved.

In this talk, I will show that certain pairs of noisy processes, each of which is useless for transmitting quantum information, can nevertheless be used together to send noiseless quantum information. This effect has no classical analog - it would be like using two disconnected phones together to place a phone call.  This mysterious behavior uncovers a rich structure in the theory of quantum communications that had not been anticipated, showing, among other things, that the quantum capacity does not completely characterize the ability of a noisy channel to transmit quantum information.

This work was published in the Sept. 26, 2008 issue of Science.