New Lower-bounds for Quantum Computation with Non-Collapsing Measurements

TL;DR

This paper establishes new lower bounds on the query complexity of quantum algorithms with non-collapsing measurements, including tight bounds for search and other problems, revealing limitations of PDQP.

Contribution

It introduces a new lower-bounding technique for PDQP, providing tighter bounds and analyzing the impact of query restrictions on quantum speed-ups.

Findings

Proves a tight a(N^{1/3})b bound on search.

Establishes tighter bounds for majority and element distinctness problems.

Shows limitations of non-adaptive queries combined with non-collapsing measurements.

Abstract

Aaronson, Bouland, Fitzsimons and Lee introduced the complexity class PDQP (which was original labeled naCQP), an alteration of BQP enhanced with the ability to obtain non-collapsing measurements, samples of quantum states without collapsing them. Although PDQP contains SZK, it still requires $\Omega(N^{1/4})$ queries to solve unstructured search. We formulate an alternative equivalent definition of PDQP, which we use to prove the positive weighted adversary lower-bounding method, establishing multiple tighter bounds and a trade-off between queries and non-collapsing measurements. We utilize the technique in order to analyze the query complexity of the well-studied majority and element distinctness problems. Additionally, we prove a tight $\Theta(N^{1/3})$ bound on search. Furthermore, we use the lower-bound to explore PDQP under query restrictions, finding that when combined with non-adaptive queries, we limit the speed-up in several cases.