Perturbative gadgets without strong interactions

TL;DR

This paper introduces a novel 2-body perturbative gadget that approximates many-body Hamiltonians using only weak interactions of order , reducing interaction strength requirements while increasing ancillary qubits and interaction terms.

Contribution

It presents a new 2-body gadget construction with weaker interaction strengths and a convergence condition for parallel application, applicable to 3- and k-body Hamiltonians.

Findings

Achieves approximation with  interaction strength instead of polynomial inverse 

Provides a convergence condition for perturbation series in parallel gadget applications

Applicable to approximating 3- and k-body Hamiltonians

Abstract

Perturbative gadgets are used to construct a quantum Hamiltonian whose low-energy subspace approximates a given quantum $k$-body Hamiltonian up to an absolute error $\epsilon$. Typically, gadget constructions involve terms with large interaction strengths of order $\text{poly}(\epsilon^{-1})$. Here we present a 2-body gadget construction and prove that it approximates a target many-body Hamiltonian of interaction strength $\gamma = O(1)$ up to absolute error $\epsilon\ll\gamma$ using interactions of strength $O(\epsilon)$ instead of the usual inverse polynomial in $\epsilon$. A key component in our proof is a new condition for the convergence of the perturbation series, allowing our gadget construction to be applied in parallel on multiple many-body terms. We also show how to apply this gadget construction for approximating 3- and $k$-body Hamiltonians. The price we pay for using much weaker interactions is a large overhead in the number of ancillary qubits, and the number of interaction terms per particle, both of which scale as $O(\text{poly}(\epsilon^{-1}))$. Our strong-from-weak gadgets have their primary application in complexity theory (QMA hardness of restricted Hamiltonians, a generalized area law counterexample, gap amplification), but could also motivate practical implementations with many weak interactions simulating a much stronger quantum many-body interaction.