New Cosystolic Expanders from Tensors Imply Explicit Quantum LDPC Codes with $Ω(\sqrt{n}\log^kn)$ Distance
Tali Kaufman, Ran J. Tessler
备注：
Added many new results: In the first version it was proven that the product of two cosystolic expanders has linear cosystoles. In this version it is proven that if one of them has an additional property, then their product is itself a cosystolic expander. It is also proven that many well known cosystolic expanders have this additional property
In this work we introduce a new notion of expansion in higher dimensions that is stronger than the well studied cosystolic expansion notion, and is termed {\em Collective-cosystolic expansion}.
We show that tensoring two cosystolic expanders yields a new cosystolic expander, assuming one of the complexes in the product, is not only cosystolic expander, but rather a collective cosystolic expander.
We then show that the well known bounded degree cosystolic expanders, the Ramanujan complexes are, in fact, collective cosystolic expanders. This enables us to construct new bounded degree cosystolic expanders, by tensoring of Ramanujan complexes.
Using our new constructed bounded degree cosystolic expanders we construct {\em explicit} quantum LDPC codes of distance $\sqrt{n} \log^k n$ for any $k$, improving a recent result of Evra et. al. \cite{EKZ}, and setting a new record for distance of explicit quantum LDPC codes.
The work of \cite{EKZ} took advantage of the high dimensional expansion notion known as cosystolic expansion, that occurs in Ramanujan complexes. Our improvement is achieved by considering tensor product of Ramanujan complexes, and using their newly derived property, the collective cosystolic expansion.
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