Optimal Hamiltonian control for variational quantum algorithms: On spin-qubit quantum processors (Girton)

Christopher K. Long, Nicholas J. Mayhall, Sophia E. Economou, Edwin Barnes, Crispin H. W. Barnes, Frederico Martins, David R. M. Arvidsson-Shukur, and Normann Mertig

Date & time: 18 September 2026 12:00–12:20 BST

Location: Girton College, University of Cambridge, Cambridge, United Kingdom (given virtually due to injury)

Conference: Foundations of Quantum Technologies  [1]

I presented a summary of Ref.  [2] and my PhD thesis  [3].

Abstract

From Ref.  [4]:

Variational quantum algorithms (VQAs) were once believed to be the fastest route to demonstrating practical quantum advantage. VQAs use a parameterized quantum circuit to perform a machine-learning task. For example, VQAs can employ the Rayleigh–Ritz method to estimate a molecule’s eigenenergies. While VQAs are already used to bootstrap quantum processors, their utility for quantum chemistry tasks has been questioned. The three main concerns are runtime, noise on near-term devices, and optimizability. In this talk, I will present a new approach to VQAs that overcomes all three concerns. To achieve this, coauthors and I replaced the gate-based quantum-circuit approach with a Hamiltonian-control approach tailored to spin-qubit quantum processors. I will present numerical emulations demonstrating a $100$-fold acceleration, along with $10^5$- and $1000$-fold improvements in the required $T_1$ and $T_2^{\ast }$ coherence times, respectively. These improvements bring the device requirements in line with present-day quantum processors. Finally, we retain the optimizability of state-of-the-art adaptive VQE algorithms through adaptive quantum optimal control and careful encoding of molecular Hamiltonians. Specifically, we ensure that the native two-qubit interactions generate the same Lie algebra as time-reversible fermionic excitations within the molecule of interest.

Recording

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Slide deck

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References

[1]

https://www.girton.cam.ac.uk/events/foundations-quantum-technologies, 2026, Internet Archive: https://web.archive.org/web/20260924164237/https://www.girton.cam.ac.uk/events/foundations-quantum-technologies. ↩

[2]

Christopher K. Long, Nicholas J. Mayhall, Sophia E. Economou, Edwin Barnes, Crispin H. W. Barnes, Frederico Martins, David R. M. Arvidsson-Shukur, and Normann Mertig. Minimal state-preparation times for silicon spin qubits, npj Quantum Information 11, 113 (2025), DOI: 10.1038/s41534-025-01027-8. ↩

[3]

Christopher K. Long. Optimal Hamiltonian control for variational quantum algorithms: on spin-qubit quantum processors, PhD thesis, University of Cambridge, 2026. ↩

[4]

Foundations of quantum technologies 2026 — book of abstracts (2026), No Internet Archive copy or cached version is available due to licensing restrictions. ↩

Footnotes

1. The files in this archive are under different licences (CC BY 4.0, Third-party all rights reserved, MIT License, SIL Open Font License 1.1), which are given for each file by the REUSE.toml in the archive and on the licences page. ↩

2. Licence of this file: CC BY 4.0 and Third-party all rights reserved; see its entry on the licences page for what each licence covers and the copyright holders. ↩