Quantum LDPC Codes with Almost Linear Minimum Distance

We give a construction of quantum LDPC codes of dimension $Θ(\log N)$ and distance $Θ(N/\log N)$ as the code length $N\to\infty$. Using a product of chain complexes this construction also provides a family of quantum LDPC codes of distance $Ω(N^{1-α/2}/\log N)$ and dimension $Ω(N^α\log N)$, where $0 \le α< 1$. We also introduce and study a new operation called lifted product, which naturally generalizes the product operations for quantum codes and chain complexes. Moreover, as a simple byproduct of our results on quantum codes, we obtain a new result on classical codes. We show that for any fixed $R < 1$ there exists an asymptotically good family of classical quasi-cyclic LDPC codes of rate at least $R$ with, in some sense, optimal circulant size $Ω(N/\log N)$ as the code length $N\to\infty$.