Shorter width truncated Taylor series for Hamiltonian dynamics simulations

TL;DR

This paper introduces a quantum circuit for Hamiltonian dynamics simulation that reduces ancilla qubit requirements exponentially by using mid-circuit measurements and circuit transformations, maintaining similar gate costs.

Contribution

It presents an alternative quantum circuit design that minimizes ancilla qubits through mid-circuit measurements and controlled transformations, improving efficiency for Hamiltonian simulations.

Findings

Exponential reduction in ancilla qubits needed

Maintains asymptotic gate cost

Implemented and tested with Guppy

Abstract

As established in the seminal work by Berry et al.[1], expanding the time evolution operator using truncated Taylor series (up to some order $K$) makes a good candidate for simulating Hamiltonian dynamics. Here, we adapt the method but present an alternative quantum circuit that maintains an equivalent asymptotic elementary gate cost but has an exponentially reduced number of ancilla qubits. This is realized by utilizing mid-circuit measurements (with early abort-and-restart of circuit execution), and transforming a series of multi-controlled$(H^k)$ to a series of singly-controlled$(H^{k'})$, where $H$ is a linear combination of unitaries and $k, k'$ are integers. The proposed circuit utilizes a total of $\lceil \log(K) \rceil + \lceil \log(L) \rceil +n$ qubits, where $L$ is the number of terms in the Hamiltonian and $n$ is the system qubit size. Our shorter width circuit with mid-measurements protocol is implemented and evaluated using the programming language Guppy[2,3].