Quantifying nonlocality: how outperforming local quantum codes is expensive

Abstract

Quantum low-density parity-check (LDPC) codes are a promising avenue to reduce the cost of constructing scalable quantum circuits. However, it is unclear how to implement these codes in practice. Seminal results of Bravyi & Terhal, and Bravyi, Poulin & Terhal have shown that quantum LDPC codes implemented through local interactions obey restrictions on their dimension $k$ and distance $d$. Here we address the complementary question of how many long-range interactions are required to implement a quantum LDPC code with parameters $k$ and $d$. In particular, in 2D we show that a quantum LDPC with distance $n^{1/2 + \epsilon}$ code requires $\Omega(n^{1/2 + \epsilon})$ interactions of length $\widetilde{\Omega}(n^{\epsilon})$. Further a code satisfying $k \propto n$ with distance $d \propto n^\alpha$ requires $\widetilde{\Omega}(n)$ interactions of length $\widetilde{\Omega}(n^{\alpha/2})$. Our results are derived using bounds on quantum codes from graph metrics. As an application of these results, we consider a model called a stacked architecture, which has previously been considered as a potential way to implement quantum LDPC codes. In this model, although most interactions are local, a few of them are allowed to be very long. We prove that limited long-range connectivity implies quantitative bounds on the distance and code dimension.