Remarkably, a team of physicists at Princeton University has been able to link individual molecules together to create a special state of quantum mechanical "entanglement." In these exotic states, these molecules remain associated with each other and can interact simultaneously, even if they are miles apart, or even if they occupy opposite ends of the universe. The research is published in the latest issue of the journal Science.
Member of the Princeton University research team. From left to right are Assistant Professor Lawrence Cheuk of the Department of Physics, Yukai Lu, a graduate student of the Department of Electrical Engineering, and Connor Holland, a graduate student of the Department of Physics. Photography: Richard Soden, Department of Physics
"This is a breakthrough in the molecular world because of the fundamental importance of quantum entanglement," said Lawrence Cheuk, assistant professor of physics at Princeton University and senior author of the paper. "But it's also a breakthrough for practical applications, as entangled molecules could become the building blocks of many future applications.
For example, these computers can solve certain problems faster than traditional computers, quantum simulators can simulate complex materials whose behavior is difficult to model, and quantum sensors can make measurements faster than traditional computers.
“One of the motivations for doing quantum science is that in the real world, it turns out that you can do better in many areas if you exploit the laws of quantum mechanics,” said Connor Holland, a graduate student in the Department of Physics.
The ability of quantum devices to outperform classical devices is called "quantum advantage." At the heart of quantum advantage are the principles of superposition and quantum entanglement. While classical computer bits can assume the value of 0 or 1, quantum bits called qubits can be in a superposition of 0 and 1 at the same time.
The latter concept, entanglement, is a major cornerstone of quantum mechanics. This occurs when two particles are inextricably linked to each other, so that the connection remains even if one particle is light-years away from the other. Albert Einstein initially questioned its validity, describing the phenomenon as "spooky behavior at a distance." Since then, physicists have shown that entanglement is, in fact, an accurate description of the physical world and the structure of reality.
"Quantum entanglement is a fundamental concept," Cheuk said, "but it's also a key factor in giving quantum advantage.
But establishing quantum advantage and achieving controllable quantum entanglement remains a challenge, not least because engineers and scientists still don't know which physical platform is best suited for creating qubits. Over the past few decades, many different technologies—such as trapped ions, photons, superconducting circuits, and more—have been explored as candidates for quantum computers and devices. The optimal quantum system or qubit platform will likely depend on the specific application.
Until this experiment, however, molecules had long defied controllable quantum entanglement. But Cheuk and his colleagues found a way to control individual molecules and coax them into these interlocked quantum states through careful manipulation in the laboratory. They also believe that molecules have certain advantages, such as those compared to atoms, that make them particularly suitable for certain applications in quantum information processing and quantum simulations of complex materials. For example, molecules have more quantum degrees of freedom than atoms and can interact in new ways.
"In practical terms, this means there are new ways to store and process quantum information," said Yukai Lu, a graduate student in electrical and computer engineering and co-author of the paper. "For example, a molecule can vibrate and rotate in multiple modes. So you can encode a qubit using two of those modes. If the molecular species are polar, then two molecules can interact even when they are spatially separated.
Still, the molecules have proven difficult to control in the laboratory due to their complexity. The freedom that makes them attractive also makes them difficult to control or fence in laboratory settings. Cheuk and his team addressed many of these challenges with a thoughtful experiment involving a complex experimental platform called a "tweezer array," in which individual molecules are picked up by a complex system of tightly focused laser beams, so-called "optical tweezers."
"Using molecules for quantum science is a new frontier, and our demonstration of on-demand entanglement is a critical step in demonstrating that molecules can be used as viable platforms for quantum science," Cheuk said.
In another article published in the same issue of Science, an independent research team led by John Doyle and Kang-Kuen Ni of Harvard University and Wolfgang Ketterle of MIT achieved similar results.
"The fact that they got the same results validates the reliability of our results," Cheuk said. "They also show that molecular tweezer arrays are emerging as an exciting new platform for quantum science.
"On-Demand Entanglement of Moleculesina Reconfigurable Optical Tweezer Array" co-authored by Connor M. Holland, Yukai Lu, and Lawrence W. Cheuk was published in Science on December 8, 2023, (DOI: 10.1126/science.adf4272). This work was supported by Princeton University, the National Science Foundation (2207518), and the Sloan Foundation (FG-2022-19104).