Efficient direct quantum state tomography using fan-out couplings

TL;DR

This paper introduces a scalable quantum state tomography method using fan-out couplings, enabling efficient reconstruction of large quantum states with noise mitigation on superconducting processors.

Contribution

The authors propose a novel direct tomography scheme combining strong-measurement estimation with fan-out architecture, achieving constant circuit depth regardless of system size.

Findings

Successfully reconstructed four-qubit states on a superconducting processor.

Estimated GHZ-state fidelity for up to 20 qubits with error mitigation.

Demonstrated efficiency and consistency with standard tomography methods.

Abstract

Characterizing quantum states is essential for validating quantum devices, yet conventional quantum state tomography becomes prohibitively expensive as system size grows. Direct tomography offers a distinct route by enabling selective access to individual complex density-matrix elements, with a particular advantage for sparse target states and some verification tasks. Here we introduce a direct quantum state tomography scheme combining strong-measurement estimation with a fan-out coupling architecture. It enables mutually commuting interactions between system qubits and a single meter qubit, thereby achieving constant circuit depth, independent of system size. Notably, the involutory fan-out coupling reduces to the identity under repetition, enabling straightforward noise scaling for quantum error mitigation. We experimentally validate the scheme on a superconducting quantum processor via the IBM Quantum Platform, demonstrating four-qubit state reconstruction and single-circuit GHZ-state fidelity estimation up to 20 qubits with error mitigation. Consistent results with standard tomography and improved efficiency establish our scheme as a promising approach to reconstructing full quantum states and scalable verification tasks.