Harnessing Molecular Complexity for Quantum Computing

A team of Harvard scientists has, for the first time, successfully trapped molecules to execute quantum operations. This breakthrough was achieved using ultra-cold polar molecules as qubits—the basic units of information driving quantum technology. Published recently in Nature, the findings pave the way for leveraging the intricate complexity of molecular structures for future advancements.

Molecules have not yet been utilized in quantum computing, despite their potential to enhance the speed of this cutting-edge technology. Their complex internal structures were long considered too intricate, fragile, and unpredictable to control, leading researchers to rely on simpler particles instead.

As a field we have been trying to do this for 20 years. And we’ve finally been able to do it!

Kang-Kuen Ni, Study Senior Co-Author and Theodore William Richards Professor, Chemistry and Physics, Center for Theory of Quantum Matter, University of Colorado

For many years, engineers and physicists have been attempting to create quantum computing. The technology promises speeds exponentially faster than those of classical computers by utilizing aspects of quantum mechanics for computation. This could lead to revolutionary advancements in a variety of fields, including science, medicine, and finance.

Experiments involving neutral atoms, superconducting circuits, and trapped ions dominate the field of quantum computing. Small individual particles can be consistently trapped in these systems to function as qubits and create quantum logic gates. The Harvard team's study outlines the more complex process needed to create an iSWAP gate—a key quantum circuit responsible for generating entanglement, the very property that underpins the immense power of quantum computing—using molecules.

The scientists began by using optical tweezers to capture sodium-cesium (NaCs) molecules in a stable, extremely cold environment. The scientists then used the electric dipole-dipole (or positive-negative) interactions between the molecules to carry out a quantum operation. The team was able to entangle two molecules and create a two-qubit Bell state, a quantum state, with 94% accuracy, by carefully manipulating how the molecules rotated relative to each other.

Similar to conventional computers, quantum computers rely on logic gates to process information. Unlike classical gates, which work with binary bits (0s and 1s), quantum gates work with qubits, which are capable of achieving what are known as superpositions, or existing in multiple states at once. This implies that quantum computers are capable of creating entangled states in the first place or even carrying out operations in multiple computational states simultaneously, which would be impossible for conventional machines.

Additionally, quantum gates are reversible and can precisely manipulate qubits while maintaining their quantum nature. In this experiment, the iSWAP gate switched the states of two qubits and applied a phase shift, which is a crucial step in creating entanglement where the states of two qubits become correlated regardless of their distance from one another.

Our work marks a milestone in trapped molecule technology and is the last building block necessary to build a molecular quantum computer. The unique properties of molecules, such as their rich internal structure, offer many opportunities to advance these technologies.

Annie Park, Study Co-Author and Postdoctoral Fellow, Center for Theory of Quantum Matter, University of Colorado

Ever since the 1990s, scientists have dreamed of using molecular systems for quantum computing because of their nuclear spins and nuclear magnetic resonance techniques. Despite promising results from several early experiments, molecules' unpredictable movements made them generally unsuitable for use in quantum operations. That may disrupt coherence, the delicate quantum state required for dependable functioning.

However, this obstacle can be addressed by trapping molecules in extremely cold environments, which allow for the control of the molecule's complex internal structures. Researchers employed optical tweezers, utilizing precisely focused lasers, to manipulate and control the quantum states of molecules while simultaneously minimizing their motion.

Physicists from the University of Colorado's Center for Theory of Quantum Matter, along with Lewis R.B. Picard, Annie J. Park, Gabriel E. Patenotte, and Samuel Gebretsadkan, were among the members of Ni's lab who helped make this discovery.

The research team examined errors resulting from any motion that did occur and measured the resulting two-qubit Bell state to assess the entire operation. The team came away from this with suggestions for enhancing the accuracy and stability of their configuration in subsequent tests. Additionally, by alternating between interacting and non-interacting states, researchers were able to digitize their experiment and gain new insights.

“There’s a lot of room for innovations and new ideas about how to leverage the advantages of the molecular platform. I’m excited to see what comes out of this,” said Ni.

The Air Force of Scientific Research, the National Science Foundation, the Physics Frontier Center, and the University Research Initiative's Multidisciplinary Research Program were among the organizations that provided funding for this study.

Journal Reference:

Ni, K., et al. (2024) Entanglement and iSWAP gate between molecular qubits. Nature. doi.org/10.1038/s41586-024-08177-3