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arXiv · 2604.04454

Efficient direct quantum state tomography using fan-out couplings

Jaekwon Chang·Guedong Park·Hyunseok Jeong·Yong Siah Teo·Yosep Kim
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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.

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Jaekwon Chang, Guedong Park, Hyunseok Jeong, Yong Siah Teo, Yosep Kim. 2026-04-06. Efficient direct quantum state tomography using fan-out couplings. https://arxiv.org/abs/2604.04454

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