We prove that the quantum code distance is NP-hard to approximate within an additive error of
cN, for some constant
c>0, where
N is the number of qubits. Our reductions are deterministic. This improves the previous square-root additive gap to
Ω(N) and resolves the explicitly stated linear-gap question of Kapshikar and Kundu. Our result holds for CSS codes with identical
X- and
Z-check spaces, and with a constant rate and constant relative distance. For every fixed
λ>1, there is a constant
c>0 such that hardness still holds even when every nonidentity stabilizer has weight greater than
λ times the quantum distance. We also improve the hardness gap of graph state distance on
N vertices of Grigorescu, Jha, and Samperton from cube-root to
Ω(N), resolving their explicitly stated open question. Both hardness results are asymptotically optimal since both distances are at most
N. Our graph state distance hardness result holds for balanced bipartite graphs with a binary adjacency matrix that is its own inverse (mod 2).
Our main technique for both hardness bounds above is classical: we show how to convert any code
C of length
m into a self-dual code
A(C) of length
N=Θ(m) while exactly doubling the original coset metric. The conversion is deterministic and efficient. We call it the metric self-dual completion of
C. It comes with a linear embedding
τ:F2m↪F2N. The embedding doubles all Hamming distances between vectors in
F2m and all pairwise distances between corresponding cosets. The embedding also guarantees that all codewords of
A(C) of weight at most
2m are exactly
τ(C).
In this paper we present six classes of topological quantum codes (TQC) on compact surfaces with genus
g≥2. These codes are derived from self-dual, quasi self-dual and denser tessellations associated with embeddings of self-dual complete graphs and complete bipartite graphs on the corresponding compact surfaces. The majority of the new classes has the self-dual tessellations as their algebraic and geometric supporting mathematical structures. Every code achieves minimum distance 3 and its encoding rate is such that
nk→1 as
n→∞, except for the one case where
nk→31 as
n→∞.
In this paper we present two new classes of binary quantum codes with minimum distance of at least three, by self-complementary self-dual orientable embeddings of voltage graphs and Paley graphs in the Galois field GF(pr), where p is a prime number and r is a positive integer. The parameters of two new classes of quantum codes are [[(2k+2)(8k+ 7); 2(8k^2+7k); d]] and [[(2k+2)(8k+9); 2(8k^2+9k+1); d]] respectively, where d>=3. For these quantum codes, the code rate approaches 1 as k goes to infinity.