Harvard Researchers Capture Molecules for Groundbreaking Quantum Computing Breakthroughs

Molecules have yet to be utilized in quantum computing, despite their capability to accelerate the ultra-high-speed experimental technology even further. Their intricate internal structures were regarded as too complex, fragile, and unpredictable to handle, leading to the use of smaller particles instead.

However, a group of Harvard researchers has achieved, for the first time, the ability to trap molecules in order to execute quantum operations. This accomplishment was made possible by employing ultra-cold polar molecules as qubits, the essential units of information that drive the technology. The results, recently released in the journal Nature, unveil new possibilities for leveraging the complexity of molecular frameworks for upcoming applications.

“As a community, we have spent the last 20 years attempting this,” noted senior co-author Kang-Kuen Ni, Theodore William Richards Professor of Chemistry and a professor of physics. “And at last, we have succeeded!”

Physicists and engineers have been striving to advance quantum computing for several decades. The technology, which takes advantage of quantum mechanics for computation, promises speeds that are exponentially more rapid than those of classical computers, potentially leading to transformative advancements in areas such as healthcare, science, and finance.

Dominating the field of quantum computing are endeavors involving trapped ions, neutral atoms, and superconducting circuits. In such systems, minute individual particles can be reliably secured to function as qubits and build quantum logic gates. The paper from the Harvard team elaborates on the significantly more intricate process necessary to utilize molecules for constructing an iSWAP gate, a crucial quantum circuit that generates entanglement — the very characteristic that enhances the power of quantum computing.

The researchers initiated their work by trapping sodium-cesium (NaCs) molecules using optical tweezers within a stable and exceptionally frigid environment. The electric dipole-dipole (or positive-negative) interactions among the molecules were subsequently deployed to conduct a quantum operation. By meticulously controlling the rotation of the molecules relative to each other, the team was able to achieve entanglement of two molecules, establishing a quantum state recognized as a two-qubit Bell state with 94 percent accuracy.

Logic gates facilitate information processing in quantum computers similarly to their function in conventional computers. Yet, while classical gates manipulate binary bits (0s and 1s), quantum gates work on qubits — which can attain what are termed superpositions, existing in multiple states at once. This means quantum computers can accomplish tasks that would be unattainable for traditional machines, such as generating entangled states in the first place — or even executing operations across multiple computational states concurrently.

Quantum gates are also reversible and can manipulate qubits with precision while maintaining their quantum characteristics. The iSWAP gate utilized in this experiment interchanged the states of two qubits and applied what is referred to as a phase shift, a vital step in inducing entanglement where the states of two qubits become correlated regardless of the distance separating them.

“Our research signifies a significant achievement in the realm of trapped molecule technology and is the final component necessary to construct a molecular quantum computer,” stated co-author and postdoctoral fellow Annie Park. “The distinct characteristics of molecules, such as their complex internal structure, provide numerous opportunities for enhancing these technologies.”

Since the 1990s, scientists have aspired to harness molecular systems, along with their nuclear spins and nuclear magnetic resonance methods, for quantum computing. A series of preliminary experiments yielded promising outcomes, but molecules were generally unstable for use in quantum operations due to their erratic movements. This unpredictability can disrupt coherence, the fragile quantum state essential for reliable operations.

However, confining molecules in ultra-cold environments, where the complex internal structures of the molecules can be regulated, assists in overcoming this obstacle. Once the researchers held these molecules with optical tweezers — employing precisely directed lasers to control tiny objects — they were able to minimize the motion of the molecules and manipulate their quantum states.

This breakthrough was made possible thanks to several members of Ni’s laboratory, including Lewis R.B. Picard, Annie J. Park, Gabriel E. Patenotte, and Samuel Gebretsadkan, in collaboration with physicists from the University of Colorado’s Center for Theory of Quantum Matter.

To assess the entirety of the procedure, the research team measured the resulting two-qubit Bell state and examined errors caused by any motion that did occur. This provided them with insights into improving the stability and precision of their setup in forthcoming experiments. Alternating between interacting and non-interacting states also allowed researchers to digitize their experiment, yielding further insights.

“There remains a substantial scope for innovations and new concepts on how to leverage the benefits of the molecular platform,” Ni remarked. “I am enthusiastic to see what develops from this.”