Exponential improvement in quantum simulations of bosons

TL;DR

This paper demonstrates an exponential speedup in quantum simulations of bosons by using the orbifold lattice Hamiltonian, which avoids exponential complexity growth in the number of qubits compared to traditional methods.

Contribution

It shows that the orbifold lattice Hamiltonian enables quantum simulations of bosons with polynomial resource scaling, unlike the exponential scaling in the Kogut-Susskind Hamiltonian.

Findings

Orbifold lattice Hamiltonian avoids exponential complexity in qubits.

Continuum limit achieved with resources scaling linearly in Q.

Traditional Kogut-Susskind Hamiltonian scales exponentially with Q.

Abstract

Hamiltonian quantum simulation of bosons on digital quantum computers requires truncating the Hilbert space to finite dimensions. The method of truncation and the choice of basis states can significantly impact the complexity of the quantum circuit required to simulate the system. For example, a truncation in the Fock basis where each boson is encoded with a register of $Q$ qubits, can result in an exponentially large number of Pauli strings required to decompose the truncated Hamiltonian. This, in turn, can lead to an exponential increase in $Q$ in the complexity of the quantum circuit. For lattice quantum field theories such as Yang-Mills theory and QCD, several Hamiltonian formulations and corresponding truncations have been put forward in recent years. There is no exponential increase in $Q$ when resorting to the orbifold lattice Hamiltonian, while we do not know how to remove the exponential complexity in $Q$ in the commonly used Kogut-Susskind Hamiltonian. Specifically, when using the orbifold lattice Hamiltonian, the continuum limit, or, in other words, the removal of the ultraviolet energy cutoff, is obtained with circuits whose resources scale like $Q$, while they scale like $\mathcal{O}(\exp(Q))$ for the Kogut-Susskind Hamiltonian: this can be seen as an exponential speed up in approaching the physical continuum limit for the orbifold lattice Hamiltonian formulation. We show that the universal framework, advocated by three of the authors (M.~H., S.~M., and E.~R.) and collaborators, provides a natural avenue to solve the exponential scaling of circuit complexity with $Q$, and it is the reason why using the orbifold lattice Hamiltonian is advantageous.