An Alternative Approach to Quantum Imaginary Time Evolution

TL;DR

This paper introduces a new quantum algorithm for imaginary-time evolution that avoids heavy classical post-processing, efficiently uses orthogonal basis states, and is suitable for nonlocal systems, demonstrated via IBM quantum simulation.

Contribution

The authors propose an alternative quantum imaginary-time evolution method that uses orthogonal basis states, reducing classical computation and enabling the study of nonlocal Hamiltonians.

Findings

Efficiently expresses the propagated state in orthogonal basis at each step.

Number of basis states needed scales linearly with qubits, controlled by precision.

Algorithm's gate complexity is polynomial in the number of qubits.

Abstract

There is increasing interest in quantum algorithms that are based on the imaginary-time evolution (ITE), a successful classical numerical approach to obtain ground states. However, most of the proposals so far require heavy post-processing computational steps on a classical computer, such as solving linear equations. Here we provide an alternative approach to implement ITE. A key feature in our approach is the use of an orthogonal basis set: the propagated state is efficiently expressed in terms of orthogonal basis states at every step of the evolution. We argue that the number of basis states needed at those steps to achieve an accurate solution can be kept of the order of $n$, the number of qubits, by controlling the precision (number of significant digits) and the imaginary-time increment. The number of quantum gates per imaginary-time step is estimated to be polynomial in $n$. Additionally, while in many QAs the locality of the Hamiltonian is a key assumption, in our algorithm this restriction is not required. This characteristic of our algorithm renders it useful for studying highly nonlocal systems, such as the occupation-representation nuclear shell model. We illustrate our algorithm through numerical implementation on an IBM quantum simulator.