MLMC-qDRIFT: Multilevel Variance Reduction for Randomized Quantum Hamiltonian Simulation

TL;DR

This paper introduces MLMC-qDRIFT, a multilevel Monte Carlo method that significantly reduces the quantum gate complexity for Hamiltonian simulation, maintaining efficiency regardless of the number of Hamiltonian terms.

Contribution

The paper develops a multilevel Monte Carlo framework for qDRIFT, reducing the gate complexity from b3 to b2 log^2(1/\u0000b5) for fixed-precision observable estimation.

Findings

Reduces total gate complexity from b3 to b2 log^2(1/b5)

Variance of level differences decays with circuit depth

Numerical experiments confirm practical gate-count savings

Abstract

Simulating quantum dynamics is one of the central applications of quantum computing. For Hamiltonians written as a sum of many terms, deterministic Trotter--Suzuki product formulas can require applying a large number of term-wise evolutions at each time step, leading to high circuit costs for large or dense systems. Randomized methods such as qDRIFT offer an alternative: each step samples only one Hamiltonian term, giving a circuit depth with no explicit dependence on the number of terms. However, when qDRIFT is used for observable estimation, high precision requires many independent random circuit realizations, resulting in a total gate complexity that scales as $\mathcal{O}(\varepsilon^{-3})$. We introduce a multilevel Monte Carlo framework for qDRIFT that reduces this sampling overhead. The method constructs a hierarchy of qDRIFT estimators with increasing circuit depths and couples adjacent levels by sharing their random Hamiltonian-term samples. This coupling makes the variance of the level differences decay with depth, allowing most samples to be taken on cheaper, coarse circuits and only a few on expensive, fine circuits. We prove that the resulting MLMC-qDRIFT estimator reduces the total gate complexity for fixed-precision observable estimation from the standard qDRIFT scaling $\mathcal{O}(\varepsilon^{-3})$ to $\mathcal{O}(\varepsilon^{-2}\log^2(1/\varepsilon))$, while preserving qDRIFT's lack of explicit dependence on the number of Hamiltonian terms. Numerical experiments for spin-chain dynamics confirm the predicted variance decay and demonstrate the practical gate-count savings of the multilevel construction.