---
tags: VQE, Hamiltonian-level, NISQ, Noise, Numerical, Theory
title: "Optimal Hamiltonian control for variational quantum algorithms: On spin-qubit quantum processors (Girton)"
sort-date: 2026-09-18
description: Foundations of Quantum Technologies | 18 September 2026 12:00–12:20 BST | Girton College, University of Cambridge, Cambridge, United Kingdom (given virtually due to injury)
---

[[/index|Christopher K. Long]]<a href="https://orcid.org/0009-0001-3230-942X"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
Nicholas J. Mayhall<a href="https://orcid.org/0000-0002-1312-9781"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
Sophia E. Economou<a href="https://orcid.org/0000-0002-1939-5589"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
Edwin Barnes<a href="https://orcid.org/0000-0003-1666-9385"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
Crispin H. W. Barnes<a href="https://orcid.org/0000-0001-7337-7245"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
Frederico Martins<a href="https://orcid.org/0000-0003-2668-2401"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
David R. M. Arvidsson-Shukur<a href="https://orcid.org/0000-0002-0185-0352"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>,
and Normann Mertig<a href="https://orcid.org/0000-0003-3025-7141"><font color="#a6ce39"><i class='fa-brands fa-orcid'></i></font></a>

**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](https://www.girton.cam.ac.uk/events/foundations-quantum-technologies) [@girton_foundations_quantum_technologies]

I presented a summary of Ref. [@Long2025] and my [[/Articles/PhD thesis]] [@thesis].

# Abstract

From Ref. [@girton_foundations_quantum_technologies_abstracts]:

> 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 $T_2^*$ 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

![[https://youtu.be/zRikXKlZ0dQ]]

# Slide deck

Download: [[HTML/Optimal Hamiltonian control for variational quantum algorithms (Girton).zip|HTML]], [[PDFs/Optimal Hamiltonian control for variational quantum algorithms (Girton).pdf|PDF]]

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