Work

Large-Scale Implementation of Quantum Subspace Expansion with Classical Shadows

Year: 2026

Type: article

Abstract: Quantum subspace expansion (QSE) offers promising avenues to perform spectral calculations on quantum processors but comes with a large measurement overhead. Informationally complete measurements, such as classical shadows, were recently proposed to overcome this bottleneck. Here, we report the first large-scale implementation of QSE with informationally complete measurements. In particular, we probe the quantum phase transition of a spin model with three-body interactions, for which we observe accurate ground state energy recovery and mitigation of local order parameters across system sizes of up to 80 qubits. We achieve this by reformulating QSE as a constrained optimization problem, obtaining rigorous statistical error estimates and avoiding numerical ill conditioning. With over 3 × 10 4 measurement basis randomizations per circuit and the evaluation of O ( 10 14 ) Pauli traces, this represents one of the most significant realizations of classical shadows to date.

Source: Physical Review Letters

DOI: 10.1103/2w21-qs38

Authors: Laurin E. Fischer, Daniel Bultrini, Timothée Dao, Ivano Tavernelli, Francesco Tacchino

Institutions: IBM (United States), École Polytechnique Fédérale de Lausanne, Heidelberg University, ETH Zurich

Language: English

Topic: Quantum Computing Algorithms and Architecture

Subfield: Artificial Intelligence

Field: Computer Science

Domain: Physical Sciences

Open Access status: hybrid

Estimated APC paid: $3,750