In this paper, we construct several new families of quantum codes with good parameters. These new quantum codes are derived from (classical) t-point (t ≥ 1) algebraic geometry (AG) codes by applying the Calderbank-Shor-Steane (CSS) construction. More precisely, we construct two classical AG codes C1 and C2 such that C1 ⊂ C2, applying after the well-known CSS construction to C1 and C2. Many of these new codes have large minimum distances when compared with their code lengths as well as they also have small Singleton defects. As an example, we construct a family [[46, 2(t2 − t1), d]] 25 of quantum codes, where t1, t2 are positive integers such that 1 < t1 < t2 < 23 and d ≥ min{46 − 2t2, 2t1 − 2}, of length n = 46, with minimum distance in the range 2 ≤ d ≤ 20, having Singleton defect at most four. Additionally, by applying the CSS construction to sequences of t-point (classical) AG codes constructed in this paper, we generate sequences of asymptotically good quantum codes. *

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Entanglement-assisted quantum-error-correcting (EAQEC) codes are quantum codes which use entanglement as a resource. These codes can provide better error correction than the (entanglement unassisted) codes derived from the traditional stabilizer formalism. In this paper, we provide a general method to construct EAQEC codes from cyclic codes. Afterwards, the method is applied to Reed–Solomon codes, BCH codes, and general cyclic codes. We use the Euclidean and Hermitian construction of EAQEC codes. Three families have been created: two families of EAQEC codes are maximal distance separable (MDS), and one is almost MDS or almost near MDS. The comparison of the codes in this paper is mostly based on the quantum Singleton bound.