Exponential Quantum Speed-ups are Generic

TL;DR

This paper demonstrates that most long quantum circuits can solve specific black-box problems exponentially faster than classical algorithms, highlighting the generic nature of quantum speed-ups.

Contribution

It shows that almost any sufficiently long quantum circuit can be used to efficiently solve black-box problems with exponential classical query complexity, using approximate unitary 3-designs.

Findings

Almost any element of an approximate unitary 3-design is useful for black-box problem solving.

Linear-sized random quantum circuits form approximate unitary 3-designs.

Quantum circuits can achieve exponential speed-ups over classical algorithms in query complexity.

Abstract

A central problem in quantum computation is to understand which quantum circuits are useful for exponential speed-ups over classical computation. We address this question in the setting of query complexity and show that for almost any sufficiently long quantum circuit one can construct a black-box problem which is solved by the circuit with a constant number of quantum queries, but which requires exponentially many classical queries, even if the classical machine has the ability to postselect. We prove the result in two steps. In the first, we show that almost any element of an approximate unitary 3-design is useful to solve a certain black-box problem efficiently. The problem is based on a recent oracle construction of Aaronson and gives an exponential separation between quantum and classical bounded-error with postselection query complexities. In the second step, which may be of independent interest, we prove that linear-sized random quantum circuits give an approximate unitary 3-design. The key ingredient in the proof is a technique from quantum many-body theory to lower bound the spectral gap of local quantum Hamiltonians.