On the Addressability Problem on CSS Codes

TL;DR

This paper investigates the limitations of implementing logical gates on subsets of qubits in CSS quantum codes, revealing fundamental no-go results for high-rate codes and exploring the role of automorphisms in addressability.

Contribution

It introduces the first systematic study of distance-preserving addressability in quantum codes, establishing impossibility results and analyzing automorphisms for high-rate CSS codes.

Findings

Impossible to implement certain logical gates on subsets in high-rate CSS codes.

Logical qubit permutation is impossible via physical permutation if rate > 1/3 and distance ≥ 3.

No-go results for CNOTs and CZs between high-rate codes under global circuit restrictions.

Abstract

Recent discoveries in asymptotically good quantum codes have intensified research on their application in quantum computation and fault-tolerant operations. This study focuses on the addressability problem within CSS codes: what circuits might implement logical gates on strict subsets of logical qubits? With some notion of fault-tolerance, we show some impossibility results: for CSS codes with non-zero rate, one cannot address a logical H, HP, PH, nor CNOT to any non-empty strict subset of logical qubits using a circuit made only from 1-local Clifford gates. Furthermore, we show that one cannot permute the logical qubits in a code purely by permuting the physical qubits, if the rate of the code is (asymptotically) greater than 1/3 and the distance is at least 3. We can show a similar no-go result for CNOTs and CZs between two such high-rate codes, albeit with a less reasonable restriction to circuits that we call ``global'' (though recent addressable CCZ gates use global circuits). This work pioneers the study of distance-preserving addressability in quantum codes, mainly by considering automorphisms of the code. This perspective offers new insights and potential directions for future research. We argue that studying this trade off between addressability and efficiency of the codes is essential to understand better how to do efficient quantum computation.