Fast-forwarding quantum evolution

TL;DR

This paper explores conditions under which certain quantum systems can be simulated more efficiently than traditionally possible, demonstrating exponential and polynomial speed-ups for various classes of Hamiltonians.

Contribution

It introduces a comprehensive definition of fast-forwarding in quantum computation and shows that multiple classes of Hamiltonians can be exponentially or polynomially fast-forwarded, expanding the scope of efficient quantum simulation.

Findings

Permutation-invariant local spin systems can be exponentially fast-forwarded.

Frustration-free local spin systems can be polynomially fast-forwarded with low-energy initial states.

Quadratic fermionic and number-conserving bosonic systems can be exponentially fast-forwarded.

Abstract

We investigate the problem of fast-forwarding quantum evolution, whereby the dynamics of certain quantum systems can be simulated with gate complexity that is sublinear in the evolution time. We provide a definition of fast-forwarding that considers the model of quantum computation, the Hamiltonians that induce the evolution, and the properties of the initial states. Our definition accounts for any asymptotic complexity improvement of the general case and we use it to demonstrate fast-forwarding in several quantum systems. In particular, we show that some local spin systems whose Hamiltonians can be taken into block diagonal form using an efficient quantum circuit, such as those that are permutation-invariant, can be exponentially fast-forwarded. We also show that certain classes of positive semidefinite local spin systems, also known as frustration-free, can be polynomially fast-forwarded, provided the initial state is supported on a subspace of sufficiently low energies. Last, we show that all quadratic fermionic systems and number-conserving quadratic bosonic systems can be exponentially fast-forwarded in a model where quantum gates are exponentials of specific fermionic or bosonic operators, respectively. Our results extend the classes of physical Hamiltonians that were previously known to be fast-forwarded, while not necessarily requiring methods that diagonalize the Hamiltonians efficiently. We further develop a connection between fast-forwarding and precise energy measurements that also accounts for polynomial improvements.