Approximate quantum error correction for correlated noise

Most of the research on quantum error-correcting codes studies an error model in which each noise operator acts on a bounded number of qubits. In this paper we study a different noise model where the noise operators act on all qubits together, but are otherwise restricted in their action. One example to such an operator is a controlled bit-flip operator, where the control depends on all qubits, i.e., we allow restricted, highly correlated noise. We show both positive and negative results. On the positive side, we show that even though controlled bit-flip errors cannot be perfectly corrected, they can be approximately corrected with a subconstant approximation error. On the negative side, we show that no nontrivial quantum error-correcting code can approximately correct controlled phase error with a subconstant approximation error.