Local random quantum circuits are approximate polynomial-designs

TL;DR

This paper proves that local random quantum circuits with a certain number of gates form approximate unitary t-designs, advancing understanding of their pseudo-randomness and applications in quantum cryptography and topological order.

Contribution

It establishes that local random quantum circuits of specific sizes are approximate t-designs, a previously unknown property for t > 3, using interdisciplinary techniques.

Findings

Local random quantum circuits form approximate unitary t-designs for t > 3.

Circuits of depth O(t^{10}n) are quantum t-copy tensor-product expanders.

Almost all circuits of size O(n^k) are indistinguishable from Haar random unitaries by smaller circuits.

Abstract

We prove that local random quantum circuits acting on n qubits composed of O(t^{10} n^2) many nearest neighbor two-qubit gates form an approximate unitary t-design. Previously it was unknown whether random quantum circuits were a t-design for any t > 3. The proof is based on an interplay of techniques from quantum many-body theory, representation theory, and the theory of Markov chains. In particular we employ a result of Nachtergaele for lower bounding the spectral gap of frustration-free quantum local Hamiltonians; a quasi-orthogonality property of permutation matrices; a result of Oliveira which extends to the unitary group the path-coupling method for bounding the mixing time of random walks; and a result of Bourgain and Gamburd showing that dense subgroups of the special unitary group, composed of elements with algebraic entries, are infty-copy tensor-product expanders. We also consider pseudo-randomness properties of local random quantum circuits of small depth and prove that circuits of depth O(t^{10}n) constitute a quantum t-copy tensor-product expander. The proof also rests on techniques from quantum many-body theory, in particular on the detectability lemma of Aharonov, Arad, Landau, and Vazirani. We give applications of the results to cryptography, equilibration of closed quantum dynamics, and the generation of topological order. In particular we show the following pseudo-randomness property of generic quantum circuits: Almost every circuit U of size O(n^k) on n qubits cannot be distinguished from a Haar uniform unitary by circuits of size O(n^{(k-9)/11}) that are given oracle access to U.