Multi-time quantum process tomography on a superconducting qubit

Christina Giarmatzi; Tyler Jones; Alexei Gilchrist; Prasanna Pakkiam; Arkady Fedorov; Fabio Costa

doi:10.22331/q-2025-12-18-1952

Multi-time quantum process tomography on a superconducting qubit

Christina Giarmatzi^1,2,3, Tyler Jones^2,4,5, Alexei Gilchrist^2,3, Prasanna Pakkiam^2,4, Arkady Fedorov^2,4, and Fabio Costa^4,6

¹School of Computer Science, University of Technology Sydney, Ultimo, Sydney, New South Wales 2007, Australia
²ARC Centre of Excellence for Engineered Quantum Systems, St. Lucia, Brisbane, Queensland 4072, Australia
³School of Mathematical and Physical Sciences, Macquarie University, Sydney, New South Wales 2122, Australia
⁴School of Maths and Physics, University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia
⁵Fiasqo, Brisbane, Queensland 4072, Australia
⁶Nordita, Stockholm University and KTH Royal Institute of Technology, Stockholm, 106 91, Sweden

Published:	2025-12-18, volume 9, page 1952
Editor:	Philip Taranto
Eprint:	arXiv:2308.00750v3
Doi:	https://doi.org/10.22331/q-2025-12-18-1952
Citation:	Quantum 9, 1952 (2025).

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Abstract

Current quantum technologies are at the cusp of becoming useful, but still face formidable obstacles such as noise. Noise severely limits the ability to scale quantum devices to the point that they would offer an advantage over classical devices. To understand the sources of noise it is necessary to fully characterise the quantum processes occurring across many time steps; only this would reveal any time-correlated noise called non-Markovian. Previous efforts have attempted such a characterisation but obtained only a limited reconstruction of such multi-time processes. In this work, we fully characterise a multi-time quantum process on superconducting hardware using in-house and cloud-based quantum processors. We achieve this by employing sequential measure-and-prepare operations combined with post-processing. Employing a recently developed formalism for multi-time processes, we detect general multi-time correlated noise. We also detect quantum correlated noise which demonstrates that part of the noise originates from quantum sources, such as physically nearby qubits on the chip.

Featured image: A quantum particle's journey: Prepared (A), Measured & Transformed (B), and Finally Measured (C), all while interacting with an environment that remembers. This results in correlated errors, and in this work, we fully characterise them in superconducting qubits.

Popular summary

All current quantum processors suffer from errors, hindering the progress toward a reliable quantum computer. Errors arise from unwanted interactions between the quantum system used to encode and manipulate information for computation and its environment. Since these errors can occur at any time during a calculation, they can be correlated—an error at the start of the calculation can cause an error at a later stage. These time-correlated errors are hard to identify, characterise, and eventually mitigate. Prior works have partially identified these correlated types of errors. In this work, we implemented for the first time a quantum protocol that fully characterises this type of errors, otherwise known as non-Markovian noise. The protocol was implemented on the University of Queensland's in-house processors and on IBM Quantum's cloud devices. Our work shows that such complete characterisation is possible in superconducting qubits, one of the most prominent quantum computing platforms, and paves the way for further characterisation mitigation strategies.

► BibTeX data

@article{Giarmatzi2025multitimequantum,
  doi = {10.22331/q-2025-12-18-1952},
  url = {https://doi.org/10.22331/q-2025-12-18-1952},
  title = {Multi-time quantum process tomography on a superconducting qubit},
  author = {Giarmatzi, Christina and Jones, Tyler and Gilchrist, Alexei and Pakkiam, Prasanna and Fedorov, Arkady and Costa, Fabio},
  journal = {{Quantum}},
  issn = {2521-327X},
  publisher = {{Verein zur F{\"{o}}rderung des Open Access Publizierens in den Quantenwissenschaften}},
  volume = {9},
  pages = {1952},
  month = dec,
  year = {2025}
}

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[1] Lee A. Rozema, Teodor Strömberg, Huan Cao, Yu Guo, Bi-Heng Liu, and Philip Walther, "Experimental aspects of indefinite causal order in quantum mechanics", Nature Reviews Physics 6 8, 483 (2024).

[2] G. A. L. White, P. Jurcevic, C. D. Hill, and K. Modi, "Unifying Non-Markovian Characterization with an Efficient and Self-Consistent Framework", Physical Review X 15 2, 021047 (2025).

[3] Kaumudibikash Goswami, Abhinash Kumar Roy, Varun Srivastava, Barr Perez, Christina Giarmatzi, Alexei Gilchrist, and Fabio Costa, "Hamiltonian characterization of multi-time processes with classical memory", New Journal of Physics 27 11, 114515 (2025).

[4] Abhinash Kumar Roy, Varun Srivastava, Soumik Mahanti, Christina Giarmatzi, and Alexei Gilchrist, "Semi-device-independent certification of quantum non-Markovianity using sequential random access codes", Physical Review A 110 1, 012608 (2024).

[5] Syed Emad Uddin Shubha and Tasnuva Farheen, "Pulse-to-Circuit Characterization of Stealthy Crosstalk Attack on Multi-Tenant Superconducting Quantum Hardware", arXiv:2509.11407, (2025).

[6] Fabio Costa, Jonathan Barrett, and Sally Shrapnel, "A de Finetti theorem for quantum causal structures", Quantum 9, 1628 (2025).

[7] Fabio Costa and Jing Yang, "Continuous operations on non-Markovian processes", arXiv:2512.05884, (2025).

[8] Abhinash Kumar Roy, Varun Srivastava, Christina Giarmatzi, and Alexei Gilchrist, "Practical Tomography of Multi-Time Processes", arXiv:2604.01482, (2026).

The above citations are from SAO/NASA ADS (last updated successfully 2026-07-14 22:55:52). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref's cited-by service no data on citing works was found (last attempt 2026-07-15 11:15:00). Could not fetch ADS cited-by data during last attempt 2026-07-15 11:15:01: Cannot retrieve data from ADS due to rate limitations.

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