Enabling quantum-computing applications

Enabling quantum-computing application

A critical challenge for quantum computing is to develop algorithms and software that can solve practical industrial problems more efficiently. Variational quantum algorithms aim at determining ground states of engineered quantum systems while classically optimizing angles in quantum gates, or more generally quantum control parameters, based on experimental measurements and feedback. These near-term quantum algorithms are presented as a viable option for relevant combinatorial problems such as quadratic unconstrained binary optimization, including the popular quantum approximate optimization algorithm for the maximum-cut graph problem. However, very little is actually known about their (guaranteed) performance, and we even lack suitable tools to analyze their operation in exponentially large spaces.

We target these challenges by developing a quantum systems theory for near-term quantum algorithms which is based on inherent symmetries and rooted in quantum control. This also includes the study of effective classical simulation techniques for high-dimensional quantum dynamics. In addition, we strive for a quantum-classical co-design of algorithms, which are studied and refined from both the quantum and the classical point of view. This provides a realistic outlook on their respective strength and weaknesses, potentially also leading to quantum-inspired algorithms.

Zoltán Zimborás, Robert Zeier, Thomas Schulte-Herbrüggen, Daniel Burgarth,
Symmetry criteria for quantum simulability of effective interactions,
Phys. Rev. A 92, 042309 (2015), doi:10.1103/PhysRevA.92.042309

David Edward Bruschi, André Xuereb, Robert Zeier,
Deciding finiteness of bosonic dynamics with tunable interactions,
J. Phys. A 58, 025204 (2024), doi:10.1088/1751-8121/ad91fc

Roberto Gargiulo, Matteo Rizzi, Robert Zeier,
Computing Classical Partition Functions: From Onsager and Kaufman to Quantum Algorithms,
ReAQCT'24, June 19-20 (2024), doi:10.1145/3665870.3665878

Sujay Kazi, Martín Larocca, Marco Farinati, Patrick J. Coles, M. Cerezo, Robert Zeier,
Analyzing the Quantum Approximate Optimization Algorithm: Ansätze, Symmetries, and Lie Algebras,
PRX Quantum 6, 040345 (2025), doi:10.1103/yfwq-yqmk
Research Highlight

Juhi Singh, Andreas Kruckenhauser, Rick van Bijnen, Robert Zeier,
Ground-state reachability for variational quantum eigensolvers: a Rydberg-atom case study,
Quantum Sci. Technol. 11, 035033 (2026), doi:10.1088/2058-9565/ae7b7b

Tim Heib, Andreea Silvia Goia, Sona Baghiyan, Robert Zeier, David Edward Bruschi,
Finite-dimensional Lie algebras in bosonic quantum dynamics: The single-mode case,
arxiv:2511.06940, doi:10.48550/arXiv.2511.06940

Roberto Gargiulo, Roberto Menta, Vittorio Giovannetti, Robert Zeier,
Obstructions to universality in globally controlled qubit graphs,
arxiv:2604.18699, doi:10.48550/arXiv.2604.18699

Roberto Gargiulo, Paul Herringer, Robert Zeier,
From Pauli Strings to Quantum Dynamics: A Unified Characterization,
arxiv:2606.09773, doi:10.48550/arXiv.2606.09773

Quantum control

Current experimental platforms for quantum computing and simulation respectively target the implementation of basic quantum gates and the simulation of the dynamics of a second quantum system that is not experimentally accessible. It is crucial that suitable control pulses are tailored to the specific experimental platform to achieve a high fidelity and robustness during its operation.

We study, model, and numerically simulate quantum devices and their dynamics. This allows us to optimize their operation while relying on methods from control and optimization theory. Beyond model-based approaches, we also develop methods to directly optimize quantum gates based on experimental feedback. Moreover, we estimate and correct for distortions resulting from electronic and optical devices utilized to control experimental platforms. We are building on cooperations with world-leading experimentalists through various third-party funded projects.

Léo Van Damme, Robert Zeier, Steffen J. Glaser, Dominique Sugny,
Application of the Pontryagin maximum principle to the time-optimal control in a chain of three spins with unequal couplings,
Phys. Rev. A 90, 013409 (2014), doi:10.1103/PhysRevA.90.013409

Juhi Singh, Robert Zeier, Tommaso Calarco, Felix Motzoi,
Compensating for Nonlinear Distortions in Controlled Quantum Systems,
Phys. Rev. Appl. 19, 064067 (2023), doi:10.1103/PhysRevApplied.19.064067

Cristina Cicali, Martino Calzavara, Eloisa Cuestas, Tommaso Calarco, Robert Zeier, Felix Motzoi,
Fast neutral-atom transport and transfer between optical tweezers,
Phys. Rev. Appl. 24, 024070 (2025), doi:10.1103/7r3w-8m61

Juhi Singh, Jan A. P. Reuter, Tommaso Calarco, Felix Motzoi, Robert Zeier,
Optimizing two-qubit gates for ultracold atoms using Fermi-Hubbard models,
Phys. Rev. Appl. 24, 034007 (2025), doi:10.1103/xqzw-m27l

Jan A. P. Reuter, Juhi Singh, Tommaso Calarco, Felix Motzoi, Robert Zeier,
Optimizing two-qubit gates for ultracold fermions in optical lattices,
Phys. Rev. A 113, 052603 (2026), doi:10.1103/j6x3-hh74

Vidisha Aggarwal, Boxi Li, Eloisa Cuestas, Tommaso Calarco, Robert Zeier, Alexei Ourjoumtsev, Felix Motzoi,
Improving single excitation fidelity in Rydberg superatoms for efficient single photon emission,
arxiv:2602.18363, doi:10.48550/arXiv.2602.18363

Matthias Hüls, Aurore A. Young, Clément Sayrin, Michel Brune, Jean-Michel Raimond, Tommaso Calarco, Felix Motzoi, Robert Zeier, Eloisa Cuestas,
Fast pulses for high-fidelity circularization of interacting Rydberg atoms,
arxiv:2607.05216, doi:10.48550/arXiv.2607.05216

FermiQP (08/2021 – 12/2025) supported by the German Federal Ministry of Education and Research through the funding program quantum technologies—from basic research to market via the project FermiQP 13N15891

HPCQS (12/2021 – 11/2025) supported by the European High-Performance Computing Joint Undertaking (JU) under grant agreement No 101018180. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and Germany, France, Italy, Ireland, Austria, Spain.

ML4Q (01/2023 – 12/2025) supported by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy – Cluster of Excellence Matter and Light for Quantum Computing (ML4Q) EXC 2004/1 – 390534769

PASQuanS2.1 (04/2023 – 09/2026) supported by the Horizon Europe programme HORIZON-CL4-2022-QUANTUM-02-SGA via the project 101113690

MUNIQC-Atoms (01/2022 – 12/2026) supported by the German Federal Ministry of Education and Research through the funding program quantum technologies—from basic research to market via the project MUNIQC-Atoms 13N16073

Vidisha Aggarwal (see here)
Dr. Eloisa Maria Cuestas (see here)
Matthias Hüls (see here)
Dr. Tobias Olsacher (July 2026)
Jan Reuter (June 2026)
Cristina Cicali (April 2026)
Sahil Ugale (Master thesis, November 2025; see here)
Dr. Juhi Singh (August 2025)
Nikkin Devaraju (Master thesis, April 2025)
Jan Nöller (Master thesis, March 2022)
Tanul Gupta (Master thesis, February 2021)