Reducing stabilizer circuits without the symplectic group

TL;DR

This paper introduces new normal forms for stabilizer circuits that simplify their structure and reduce gate counts by avoiding reliance on the symplectic group decomposition, with practical tests confirming efficiency gains.

Contribution

It provides two novel normal forms for stabilizer circuits using simple conjugation rules, improving circuit simplification without symplectic group decomposition.

Findings

Second normal form reduces controlled-Z gates to at most n

Normal forms are applicable to stabilizer and graph states

Experimental results show reduced gate count in practice

Abstract

We start by studying the subgroup structures underlying stabilizer circuits. Then we apply our results to provide two normal forms for stabilizer circuits. These forms are computed by induction using simple conjugation rules in the Clifford group and our algorithms do not rely on a special decomposition in the symplectic group. The first normal form has shape CX-CZ-P-Z-X-H-CZ-P-H, where CX (resp. CZ) denotes a layer of CNOT (resp. controlled-Z) gates, P a layer of phase gates, X (resp. Z) a layer of Pauli-X (resp. Pauli-Z) gates. Then we replace most of the controlled-Z gates by CNOT gates to obtain a second normal form of type P-CX-CZ-CX-Z-X-H-CZ-CX-P-H. In this second form, both controlled-Z layers have depth 1 and together contain therefore at most n controlled-Z gates. We also consider normal forms for stabilizer states and graph states. Finally we carry out a few tests on classical and quantum computers in order to show experimentally the utility of these normal forms to reduce the gate count of a stabilizer circuit.