Controlling neutral atoms and enabling quantum-computing applications
Our research explores the full range of the quantum-computing stack from hardware-centric quantum control engineering to near-term quantum algorithms. In one focus area, we engineer and optimize control pulses for quantum computers and simulators at the interface to experimental platforms based on neutral and Rydberg atoms. We support the design of robust, high-precision control firmware adapted to ever more powerful quantum computing devices.
In a second focus area, we aim at enabling quantum-computing applications by studying limitations and opportunities for near-term quantum algorithms and their capacity for scientific and industrial applications such as in quantum simulation. These two focus areas are connected by the development of a quantum systems theory which is also based on the inherent symmetries of high-dimensional quantum dynamics.
Controlling neutral atoms
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. This is our challenge with a primary focus on experimental platforms based on neutral and Rydberg atoms.
Current tasks include the controlled transport of neutral atoms with tweezers and the implementation of two-qubit collision gates. To this end, 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 the experimental platform. We are building on cooperations with world-leading experimentalists through various third-party funded projects.
Publications
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. Applied 19, 064067 (2023), doi: 10.1103/PhysRevApplied.19.064067
Enabling quantum computing applications
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.
Publications
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
Sujay Kazi, Martín Larocca, Marco Farinati, Patrick J. Coles, M. Cerezo, Robert Zeier,
Analyzing the quantum approximate optimization algorithm: ansätze, symmetries, and Lie algebras,
arXiv:2410.05187, doi:10.48550/arXiv.2410.05187