Error correction schemes for fully correlated quantum channels
  protecting both quantum and classical information
We study efficient quantum error correction schemes for the fully correlated
channel on an $n$-qubit system with error operators that assume the form
$\sigma_x^{\otimes n}$, $\sigma_y^{\otimes n}$, $\sigma_z^{\otimes n}$.
Previous schemes are improved to facilitate implementation. In particular, when
$n$ is odd and equals $2k+1$, we describe a quantum error correction scheme
using one arbitrary qubit $\sigma$ to protect the data state $\rho$ in a
$2k$-qubit system. The encoding operation $\sigma \otimes \rho \mapsto
\Phi(\sigma \otimes \rho)$ only requires $3k$ CNOT gates (each with one control
bit and one target bit). After the encoded state $\Phi(\sigma\otimes \rho)$
goes through the channel, we can apply the inverse operation $\Phi^{-1}$ to
produce $\tilde \sigma \otimes \rho$ so that a partial trace operation can
recover $\rho$. When $n$ is even and equals $2k+2$, we describe a hybrid
quantum error correction scheme using any one of the two classical bits $\sigma
\in \{|ij\rangle \langle ij|: i, j \in \{0,1\}\}$ to protect a $2k$-qubit state
$\rho$ and 2 classical bits. The encoding operation $\sigma \otimes \rho
\mapsto \Phi(\sigma \otimes \rho)$ can be done by $3k+2$ CNOT gates and a
single quibt Hadamard gate. After the encoded state $\Phi(\sigma\otimes \rho)$
goes through the channel, we can apply the inverse operation $\Phi^{-1}$ to
produce $\sigma \otimes \rho$ so that a perfect protection of the two classical
bits $\sigma$ and the $2k$-qubit state is achieved. If one uses an arbitrary
$2$-qubit state $\sigma$, the same scheme will protect $2k$-qubit states. The
scheme was implemented using Matlab, Mathematica, Python, and the IBM's quantum
computing framework qiskit.
Chi-Kwong Li Seth Lyles Yiu-Tung Poon