Fast and Effective Techniques for T-Count Reduction via Spider Nest Identities

Authors Niel de Beaudrap , Xiaoning Bian, Quanlong Wang

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Author Details

Niel de Beaudrap
  • Department of Computer Science, University of Oxford, United Kingdom
Xiaoning Bian
  • Department of Mathematics & Statistics, Dalhousie University, Halifax, Canada
Quanlong Wang
  • Department of Computer Science, University of Oxford, United Kingdom
  • Cambridge Quantum Computing Ltd., Cambridge, United Kingdom


We thank Earl Campbell, Luke Heyfron, Alexander Cowtan, Aleks Kissinger, and John van de Wetering for helpful discussions. We extend a very special thanks to Matthew Amy, who wrote a small extension of feynver [Matthew Amy, 2018] to allow verification of procedures which post-select the |+> state, for the express purpose of helping us to independently verify the correctness of reductions such as appear in this work and in Ref. [Niel de Beaudrap et al., 2019]. X. Bian would like to thank his Ph.D. supervisor Peter Selinger for his support.

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Niel de Beaudrap, Xiaoning Bian, and Quanlong Wang. Fast and Effective Techniques for T-Count Reduction via Spider Nest Identities. In 15th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2020). Leibniz International Proceedings in Informatics (LIPIcs), Volume 158, pp. 11:1-11:23, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2020)


In fault-tolerant quantum computing systems, realising (approximately) universal quantum computation is usually described in terms of realising Clifford+T operations, which is to say a circuit of CNOT, Hadamard, and π/2-phase rotations, together with T operations (π/4-phase rotations). For many error correcting codes, fault-tolerant realisations of Clifford operations are significantly less resource-intensive than those of T gates, which motivates finding ways to realise the same transformation involving T-count (the number of T gates involved) which is as low as possible. Investigations into this problem [Matthew Amy et al., 2013; Gosset et al., 2014; Matthew Amy et al., 2014; Matthew Amy et al., 2018; Earl T. Campbell and Mark Howard, 2017; Matthew Amy and Michele Mosca, 2019] has led to observations that this problem is closely related to NP-hard tensor decomposition problems [Luke E. Heyfron and Earl T. Campbell, 2018] and is tantamount to the difficult problem of decoding exponentially long Reed-Muller codes [Matthew Amy and Michele Mosca, 2019]. This problem then presents itself as one for which must be content in practise with approximate optimisation, in which one develops an array of tactics to be deployed through some pragmatic strategy. In this vein, we describe techniques to reduce the T-count, based on the effective application of "spider nest identities": easily recognised products of parity-phase operations which are equivalent to the identity operation. We demonstrate the effectiveness of such techniques by obtaining improvements in the T-counts of a number of circuits, in run-times which are typically less than the time required to make a fresh cup of coffee.

Subject Classification

ACM Subject Classification
  • Computer systems organization → Quantum computing
  • T-count
  • Parity-phase operations
  • Phase gadgets
  • Clifford hierarchy
  • ZX calculus


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