Deriving Grover's lower bound from simple physical principles

TL;DR

This paper demonstrates that Grover's quadratic speed-up in search algorithms is unaffected by the order of interference in physical theories, indicating that higher interference does not enhance computational power beyond quantum limits.

Contribution

The work shows that Grover's lower bound holds regardless of the interference order, challenging the idea that more interference could lead to faster quantum algorithms.

Findings

Grover's quadratic lower bound is independent of interference order.

Post-quantum interference does not provide additional computational speed-up.

Quantum interference limits are fundamental to the search problem's complexity.

Abstract

Grover's algorithm constitutes the optimal quantum solution to the search problem and provides a quadratic speed-up over all possible classical search algorithms. Quantum interference between computational paths has been posited as a key resource behind this computational speed-up. However there is a limit to this interference, at most pairs of paths can ever interact in a fundamental way. Could more interference imply more computational power? Sorkin has defined a hierarchy of possible interference behaviours---currently under experimental investigation---where classical theory is at the first level of the hierarchy and quantum theory belongs to the second. Informally, the order in the hierarchy corresponds to the number of paths that have an irreducible interaction in a multi-slit experiment. In this work, we consider how Grover's speed-up depends on the order of interference in a theory. Surprisingly, we show that the quadratic lower bound holds regardless of the order of interference. Thus, at least from the point of view of the search problem, post-quantum interference does not imply a computational speed-up over quantum theory.