March 03, 2025 -- Researchers at the Max Planck Institute of Quantum Optics (MPQ), in collaboration with the chemicals company Covestro, have developed a new method for simulating chemical models using fermionic quantum simulators. The key advantage: The energetic states prepared in the lab obey the same laws as the electrons in molecules – which directly align with the molecular behaviour being simulated. By successfully mapping quantum chemistry algorithms onto their fermionic quantum simulator, the team has taken a significant step towards leveraging quantum computing for fundamental questions in chemistry.

Accurately simulating molecular chemistry could have far-reaching implications for fields such as pharmaceuticals, materials, and energy. However, classical computing methods struggle to model larger and more complex molecules, especially when predicting ground-state energies and reaction pathways. Quantum simulators, though better suited for these challenges in theory, come with their own difficulties: A key issue is representing the fermionic nature of electrons interacting with each other. Fermions, the particles that make up matter, obey specific symmetries that are computationally challenging to encode.

This is the challenge the researchers from MPQ and Covestro addressed. The team found a way to map quantum chemistry algorithms onto fermionic quantum simulators – quantum systems based on neutral ultracold atoms in optical lattices. Unlike spin-based quantum computers, these simulators naturally respect fermionic symmetries, making it easier to simulate complex molecules accurately.

Automating Quantum Circuit Generation

Central to their approach is an automated system that converts a molecular Hamiltonian – the mathematical representation of a molecule’s energy landscape – to a quantum circuit, a sequence of operations executable on a quantum simulator. The researchers found that established quantum chemistry algorithms can be easily mapped onto fermionic platforms, utilising naturally occurring interactions like local interaction and atomic motion.

By applying a few dozen interaction and tunnelling pulses to a simple starting state in an optical lattice, the "Ansatz" – a carefully chosen sequence of gates – produces a quantum state similar to the ground state of the target molecule. The ground state energy can then be refined by further tweaking the pulse sequence. Test circuits were successfully developed for small molecules, including H4, H2O, and HF, optimised for current and emerging hardware at MPQ.

Bringing Quantum Chemistry to Life

While the new method marks a major step forward, two key challenges remain before it can be applied to real-world chemical problems. First, the team must demonstrate that fermionic motional gates with high fidelity can be achieved locally at the scale of individual lattice sites. Second, data rates need to significantly increase to handle industrially relevant optimization tasks within practical timeframes.

Despite these hurdles, the researchers’ work offers a clear roadmap for bringing molecular simulations with ultracold atoms closer to reality. “We have provided detailed error estimates and practical steps for implementing these simulations in the lab. This roadmap could help speed up the development of quantum chemistry simulations in real-world settings,” notes Philipp Preiss, senior author of the study. Upcoming experiments will focus on enhancing gate fidelity and increasing simulation accuracy to model more intricate molecular systems over the next few years.

Quantum physics and quantum chemistry working together

Philipp Preiss highlights the value of the interdisciplinary collaboration:  "It was a challenge to find common ground at first, but once we identified the right connections between theory and experiment, everything came together quickly. This project shows the huge potential for quantum simulators in chemistry."

Looking ahead, the MPQ team hopes their method will soon enable accurate simulations of complex molecular systems using ultracold atoms, potentially transforming applications in drug development, material design, and sustainable energy solutions.