Near-perfect Reachability of Variational Quantum Search with Depth-1 Ansatz

TL;DR

This paper demonstrates that a variational quantum search algorithm with a simple depth-1 Ansatz can achieve near-perfect reachability for any number of qubits, significantly advancing its potential to outperform Grover's algorithm and solve NP-complete problems.

Contribution

It proves that the Variational Quantum Search with a depth-1 Ansatz has near-perfect reachability for any number of qubits, establishing its exponential advantage over Grover's algorithm.

Findings

VQS with depth-1 Ansatz achieves near-perfect reachability.

Reachability exponentially improves with the number of qubits.

Numerical studies validate the theoretical results.

Abstract

Grover's search algorithm is renowned for its dramatic speedup in solving many important scientific problems. The recently proposed Variational Quantum Search (VQS) algorithm has shown an exponential advantage over Grover's algorithm for up to 26 qubits. However, its advantage for larger numbers of qubits has not yet been proven. Here we show that the exponentially deep circuit required by Grover's algorithm can be replaced by a multi-controlled NOT gate together with either a single layer of Ry gates or two layers of circuits consisting of Hadamard and NOT gates, which is valid for any number of qubits greater than five. We prove that the VQS, with a single layer of Ry gates as its Ansatz, has near-perfect reachability in finding the good element of an arbitrarily large unstructured data set, and its reachability exponentially improves with the number of qubits, where the reachability is defined to quantify the ability of a given Ansatz to generate an optimal quantum state. Numerical studies further validate the excellent reachability of the VQS. Proving the near-perfect reachability of the VQS, with a depth-1 Ansatz, for any number of qubits completes an essential step in proving its exponential advantage over Grover's algorithm for any number of qubits, and the latter proving is significant as it means that the VQS can efficiently solve NP-complete problems.