Logarithmic-Depth Quantum Circuits for Hamming Weight Projections
Abstract
A pure state of fixed Hamming weight is a superposition of computational basis states such that each bitstring in the superposition has the same number of ones. Given a Hilbert space of the form \( \mathcal{H} = (\mathbb{C}_2)^{\otimes n}\), or an \(n\)-qubit system, the identity operator can be decomposed as a sum of projectors onto subspaces of fixed Hamming weight. In this work, we propose several quantum algorithms that realize a coherent Hamming weight projective measurement on an input pure state, meaning that the post-measurement state of the algorithm is the projection of the input state onto the corresponding subspace of fixed Hamming weight. We analyze a depth-width trade-off for the corresponding quantum circuits, allowing for a depth reduction of the circuits at the cost of more control qubits. For an \(n\)-qubit input, the depth-optimal algorithm uses \(\mathcal{O}(n)\) control qubits and the corresponding circuit has depth \( \mathcal{O}(\operatorname{log}(n)) \), assuming that we have the ability to perform qubit resets. Furthermore, the proposed algorithm construction uses only one- and two-qubit gates.

BibTeX
@article{PhysRevA.110.052401,
title = {Logarithmic-depth quantum circuits for Hamming weight projections},
author = {Rethinasamy, Soorya and LaBorde, Margarite L. and Wilde, Mark M.},
journal = {Phys. Rev. A},
volume = {110},
issue = {5},
pages = {052401},
numpages = {14},
year = {2024},
month = {Nov},
publisher = {American Physical Society},
doi = {10.1103/PhysRevA.110.052401},
url = {https://link.aps.org/doi/10.1103/PhysRevA.110.052401}
}