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arXiv:2411.05404 · Quantum Information Processing

Theory and experimental demonstration of Wigner tomography of unknown unitary quantum gates

Amit Devra, Léo Van Damme, Frederik vom Ende, Emanuel Malvetti, Steffen J. Glaser

Published Jul 27, 2026• 1 citation
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Abstract

Abstract We investigate the tomography of unknown unitary quantum processes within the framework of a finite-dimensional Wigner-type representation. This representation provides a rich visualization of quantum operators by depicting them as shapes assembled as a linear combination of spherical harmonics. These shapes can be experimentally tomographed using a scanning-based phase-space tomography approach. However, so far, this approach was limited to known target processes and only provided information about the controlled version of the process rather than the process itself. To overcome this limitation, we introduce a general protocol to extend Wigner tomography to unknown unitary processes. This new method enables experimental tomography by combining a set of experiments with classical post-processing algorithms introduced herein to reconstruct the unknown process. We also demonstrate the tomography approach experimentally on IBM quantum devices and present the specific calibration circuits required for quantifying undesired errors in the measurement outcomes of these demonstrations.

DOI
10.1007/s11128-026-05269-z

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The successful implementation of complex quantum algorithms depend crucially on our ability to determine unknown quantum states and operations. In this work, we present adaptive and non-adaptive methods to determine a 1-qubit unitary gate with 5 and 6 Pauli measurements. We demonstrate the method on the Bloch sphere to show that studying higher dimensional real space may help in finding tomography methods of multiple qubit unitary gates. Next, we show an adaptive method to uniquely determine a general d-dimensional pure state among all quantum states with at most 2d - 1 measurements. This method is then applied to determine a general d-dimensional unitary gate with at most d^2 + d - 1 measurements. These methods are applied in tomographing a 2-qubit universal gate set with five unitary gates. On the NMR experimental system, the lowest fidelity achieved was above 97% with 42 Pauli measurements, comparing to 99% using traditional method that requires 240 Pauli measurements.