CLASSICAL SIMULATION OF QUANTUM CIRCUITS BY HALF GAUSS SUMS † ‡ KAIFENG BU ∗ AND DAX ENSHAN KOH ABSTRACT. We give an efficient algorithm to evaluate a certain class of expo- nential sums, namely the periodic, quadratic, multivariate half Gauss sums. We show that these exponential sums become #P-hard to compute when we omit either the periodic or quadratic condition. We apply our results about these exponential sums to the classical simulation of quantum circuits, and give an alternative proof of the Gottesman-Knill theorem. We also explore a connection between these exponential sums and the Holant framework. In particular, we generalize the existing definition of affine signatures to arbitrary dimensions, and use our results about half Gauss sums to show that the Holant problem for the set of affine signatures is tractable. CONTENTS 1. Introduction 1 2. Half Gausssums 5 3. m-qudit Clifford circuits 11 4. Hardness results and complexity dichotomy theorems 15 5. Tractable signature in Holant problem 18 6. Concluding remarks 20 Acknowledgments 20 Appendix A. Exponential sum terminology 20 Appendix B. Properties of Gauss sum 21 Appendix C. Half Gauss sum for ξd = ω2d with even d 21 Appendix D. Relationship between half− Gauss sums and zeros of a polynomial 22 References 23 arXiv:1812.00224v1 [quant-ph] 1 Dec 2018 1. INTRODUCTION Exponential sums have been extensively studied in number theory [1] and have a rich history that dates back to the time of Gauss [2]. They have found numerous (†) SCHOOL OF MATHEMATICAL SCIENCES, ZHEJIANG UNIVERSITY, HANGZHOU, ZHE- JIANG 310027, CHINA (*) DEPARTMENT OF PHYSICS, HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS 02138, USA (‡) DEPARTMENT OF MATHEMATICS, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS 02139, USA E-mail addresses:
[email protected] (K.Bu),
[email protected] (D.E.Koh).