  In this paper, we introduce a unified framework to construct
entanglement-assisted quantum error-correcting codes, including additive and
nonadditive codes, based on the codeword stabilized framework on subsystems.
The codeword stabilized (CWS) framework is a scheme to construct quantum
error-correcting codes (QECCs) including both additive and nonadditive codes,
and gives a method to construct a QECC from a classical error-correcting code
in standard form. Entangled pairs of qubits (ebits) can be used to improve
capacity of quantum error correction. In addition, it gives a method to
overcome the dual-containing constraint. Operator quantum error correction
(OQEC) gives a general framework to construct quantum error-correcting codes.
We construct OQEC codes with ebits based on the CWS framework. This new scheme,
entanglement-assisted operator codeword stabilized (EAOCWS) quantum codes, is
the most general framework we know of to construct both additive and
nonadditive codes from classical error-correcting codes. We describe the
formalism of our scheme, demonstrate the construction with examples, and give
several EAOCWS codes.
