Constant depth fault-tolerant Clifford circuits for multi-qubit large block codes

Abstract

Fault-tolerant quantum computation (FTQC) schemes using large block codes that encodek> 1 qubits innphysical qubits can potentially reduce the resource overhead to a great extent because of their high encoding rate. However, the fault-tolerant (FT) logical operations for the encoded qubits are difficult to find and implement, which usually takes not only a very large resource overhead but also longin situcomputation time. In this paper, we focus on Calderbank-Shor-Steane [[n,k,d]] (CSS) codes and their logical FT Clifford circuits. We show that the depth of an arbitrary logical Clifford circuit can be implemented fault-tolerantly inO(1) stepsin situvia either Knill or Steane syndrome measurement circuit, with the qualified ancilla states efficiently prepared. Particularly, for those codes satisfyingk/n similar to Theta(1), the resource scaling for Clifford circuits implementation on the logical level can be the same as on the physical level up to a constant, which is independent of code distanced. With a suitable pipeline to produce ancilla states, our scheme requires only a modest resource cost in physical qubits, physical gates, and computation time for very large scale FTQC.