The Quartic Gap Between Quantum and Dequantized Algorithms for Linear Algebra Is Tight

Abstract

Quantum algorithms for linear algebra that access a matrix $A$ through quantum random access memory (QRAM) prepare quantum states proportional to the solutions of linear systems and related problems. Dequantized classical algorithms, when given sampling and query access to $A$, return sampling and query access to an approximate solution. Both kinds of algorithms use a number of queries that is polynomial in the normalized Frobenius norm $ρ=\|A\|_F/\|A\|$ and does not directly depend on the dimension. For linear systems at fixed condition number and accuracy, quantum algorithms use $\widetilde O(ρ)$ queries, whereas the best dequantized algorithms use $\widetilde O(ρ^4)$. We prove that this quartic gap is tight in the worst case, up to a logarithmic factor. For every constant $κ_0>1$, we construct dense linear systems with condition number at most $κ_0$ for which sampling from the solution within a sufficiently small constant total variation distance requires $Ω(ρ^4/\logρ)$ classical queries but only $Θ(ρ)$ quantum queries. The same separation holds, for errors below smaller thresholds, for positive-definite systems, least squares, ridge regression, matrix powers and square roots, imaginary-time and real-time Hamiltonian evolution, and sampling from the top principal component. Our results therefore show that many quantum algorithms for linear algebra, achieve a near-quartic speedup in the worst case, over classical algorithms with sampling and query access. All of these classical lower bounds follow from the same framework, which is based on a lower bound for sampling from distributions that depend on a fixed set of Fourier coefficients of an unknown Boolean function.