The Need for Structure in Quantum LDPC Codes

Understand

Existence of quantum low-density parity-check (LDPC) codes whose minimal distance scales linearly with the number of qubits is a major open problem in quantum information.

• Its practical interest stems from the need to protect information in a future quantum computer, and its theoretical appeal relates to a deep "global-to-local" notion in quantum mechanics: whether we can constrain long-range entanglement using local checks.
• Given the inability of lattice-based quantum LDPC codes to achieve linear distance, research has recently shifted to the other extreme end of topologies, so called high-dimensional expanders.
• In this work we show that trying to leverage the mere "random-like" property of these expanders to find good quantum codes may be futile: quantum CSS codes of $n$ quits built from $d$-complexes that are $\varepsilon$-far from perfectly random, in a well-known sense called discrepancy, have a small minimal distance.