Princeton physicists have successfully linked single molecules for the first time

In the work that can lead to more powerful Quantitative statisticsResearchers from Princeton University have succeeded in forcing molecules into quantum entanglement.

For the first time, a team of physicists at Princeton University has been able to bind individual particles together in special “entangled” states of quantum mechanics. In these strange states, particles remain bound together, and can interact simultaneously, even if they are miles apart, or even if they occupy opposite ends of the universe. This research was published in the journal Sciences.

Molecular entanglement: a breakthrough in practical applications

“This is a breakthrough in the world of molecules because of the fundamental importance of quantum entanglement,” said Lawrence Cheok, an assistant professor of physics at UCLA. Princeton University and lead author of the paper. “But it is also a major breakthrough for practical applications because cross-linked molecules could be the building blocks for many future applications.”

These include, for example, quantum computers that can solve certain problems much faster than classical computers, quantum simulators that can model complex materials whose behaviors are difficult to model, and quantum sensors that can measure faster than their classical counterparts.

Laser setup to cool, control, and crosslink individual molecules. Credit: Richard Soden, Department of Physics, Princeton University

“One of the motivations for doing quantum science is that in the practical world it turns out that if you harness the laws of quantum mechanics, you can do much better in many areas,” says Connor Holland, a graduate student in the Department of Physics. And co-author of the work.

The ability of quantum devices to outperform classical devices is known as quantum advantage. At the heart of quantum advantage are the principles of superposition and quantum entanglement. While a classical computer bit can take on the value of either 0 or 1, quantum bits, called qubits, can simultaneously be in a state of superposition of 0 and 1. The latter concept, entanglement, is a major cornerstone of quantum mechanics, and occurs When two particles become so closely connected to each other, this connection persists, even if one of the particles is light-years away from the other. This phenomenon was described by Albert Einstein, who initially doubted its authenticity, as “ghostly action at a distance.” Since then, physicists have proven that entanglement is actually an accurate description of the physical world and how reality is structured.

Challenges and progress in quantum entanglement

“Quantum entanglement is a fundamental concept, but it is also the key element that gives quantum advantage,” Cheok says.

But building a quantum advantage and achieving controllable quantum entanglement remains a challenge, not least because engineers and scientists are still unclear about which physical platform is best for creating qubits. In the past decades, many different technologies – such as trapped ions, photons, and superconducting circuits, to name a few – have been explored as candidates for quantum computers and devices. The optimal quantum system or qubit platform can depend well on the specific application.

But until this experiment, molecules had long defied controllable quantum entanglement. But Cheok and his colleagues have found a way, through precise manipulation in the laboratory, to control individual molecules and coax them into these entangled quantum states. They also believed that molecules had certain advantages – over atoms, for example – that made them particularly suitable for certain applications in quantum information processing and quantum simulations of complex materials. Compared to atoms, for example, molecules have greater quantum degrees of freedom and can interact in new ways.

“What this means, in practical terms, is that there are new ways to store and process quantum information,” said Yukai Lu, a graduate student in electrical and computer engineering and a co-author of the paper. “For example, a molecule can vibrate and rotate in multiple modes. So, you can use two of these modes to encode a qubit. If a molecular Classify If it is polar, two molecules can interact even when they are spatially separated.

However, it has proven very difficult to control molecules in the laboratory due to their complexity. The degrees of freedom that make them attractive also make them difficult to control or adapt in laboratory settings.

Innovative experimental techniques and future prospects

Cheok and his team addressed many of these challenges through a carefully considered experiment. They first chose a molecular species that is polar and can be cooled by lasers. They then cooled the molecules with a laser to ultracold temperatures where quantum mechanics takes center stage. Individual molecules were then captured by a complex system of highly focused laser beams, called “optical tweezers.” By engineering the tweezer sites, they were able to create large arrays of single molecules and place them individually in any desired one-dimensional configuration. For example, they created isolated pairs of molecules and also defect-free chains of molecules.

Next, they encoded the qubit in the spin-free state of the molecule. They were able to show that this molecular qubit remained coherent, that is, that it remembered its superposition. In short, researchers have demonstrated the ability to create coherent, well-controlled qubits from individually controlled particles.

To entangle the molecules, they had to make the molecule interact. Using a series of microwave pulses, they were able to make individual molecules interact with each other in a coherent manner. By allowing the reaction to continue for a specified period of time, they were able to implement a two-qubit gate that entangled two molecules. This is important because such a two-qubit entangled gate is a fundamental building block for both universal digital quantum computing and the simulation of complex materials.

The potential of this research to study different areas of quantum science is great, given the innovative features offered by this new platform for molecular tweezer arrays. In particular, the Princeton team is interested in exploring the physics of many interacting particles, which can be used to simulate quantum many-body systems where interesting emergent behavior such as new forms of magnetism could emerge.

“Using molecules in quantum science is a new frontier, and our demonstration of on-demand entanglement is a key step in demonstrating that molecules can be used as a viable platform in quantum science,” Cheok said.

In a separate article published in the same issue of SciencesAn independent research group led by John Doyle and Kang Kuen Ni at Harvard University and Wolfgang Ketterle at MIT achieved similar results.

“The fact that they got the same results confirms the reliability of our results,” Cheok said. “They also show that molecular tweezer arrays are becoming an exciting new platform for quantum science.”

Reference: “On-demand entanglement of molecules in a reconfigurable optical tweezer array” by Connor M. Holland, Yukai Lu, and Lawrence W. Cheok, 7 December 2023, Sciences.
doi: 10.1126/science.adf4272

The work was supported by Princeton University, the National Science Foundation (Grant No. 2207518), and the Sloan Foundation (Grant No. FG-2022-19104).