In a remarkable first, a team of physicists at Princeton has succeeded in binding together individual molecules in special states that are quantum mechanically “entangled.” In these strange states, the molecules remain bound to each other – and can interact simultaneously – even if they are miles apart or even if they occupy opposite ends of the universe. This research is published in the current issue of the journal Science.
“This is a breakthrough in the world of molecules because of the fundamental importance of quantum entanglement,” said Lawrence Check, assistant professor of physics at Princeton University, senior author of the paper and a 2010 graduate of Princeton’s course. “But it’s also a breakthrough for practical applications, as entangled molecules can be the building blocks for many future applications.
These include, for example, quantum computers that can solve certain problems much faster than conventional computers, quantum simulators that can model complex materials whose behavior is difficult to model, and quantum sensors that can measure faster from their traditional counterparts.
“One of the reasons to do 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 fields,” said 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 the “quantum advantage”. And at the core of the quantum advantage are the principles of superposition and quantum entanglement. While a classical computer bit can assume the value of 0 or 1, quantum bits, called qubits, can simultaneously be in a superposition of 0 and 1.
The latter concept, entanglement, is a fundamental cornerstone of quantum mechanics. This happens when two particles become inextricably bound to each other, so that this bond continues even if one particle is light-years away from the other particle. This is the phenomenon that Albert Einstein, who initially questioned its validity, described as “spooky action at a distance.” Physicists have since demonstrated that entanglement is in fact an accurate description of the physical world and how reality is structured.
“Quantum entanglement is a fundamental concept,” Cheuk said, “but it’s also the key ingredient that gives quantum edge.”
But building a quantum edge and achieving controllable quantum entanglement remain challenging, not least because engineers and scientists are still unclear about which physical platform is best for creating qubits. In recent decades, many different technologies—such as trapped ions, photons, superconducting circuits, to name a few—have been explored as candidates for quantum computers and devices. The optimal quantum system or qubit platform may very well depend on the particular application.
Until this experiment, however, molecules had long resisted controllable quantum entanglement. But Cheuk and his colleagues found a way, through careful manipulation in the lab, to control individual molecules and force them into these interconnected quantum states. They also believe that molecules have certain advantages — over atoms, for example — that make them particularly well-suited for certain applications in quantum information processing and quantum simulation of complex materials. Compared to atoms, for example, molecules have more quantum degrees of freedom and can interact in new ways.
“This effectively means that 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 use two of these modes to encode a qubit. If the molecular species is polar, two molecules can interact even when they are spatially separated.
Nevertheless, the molecules have proven extremely difficult to control in the laboratory due to their complexity. The very degrees of freedom that make them attractive also make them difficult to control or confine in a laboratory setting. Cheuk and his team addressed many of these challenges through a carefully thought-out experiment involving a sophisticated experimental platform known as “array of tweezers,”, in which individual molecules were captured 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 key step in demonstrating that molecules can be used as a viable platform for quantum science,” Cheuk said.
In a separate article published in the same issue of Sciencean independent research group 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 confirms the reliability of our results,” Cheuk said. “They also show that arrays of molecular tweezers are becoming an exciting new platform for quantum science.”
“On-Demand Entanglement of Molecules in a Reconfigurable Optical Tweezer Array” by Connor M. Holland, Yukai Lu, and Lawrence W. Cheuk was published in Science on December 8, 2023 (DOI: 10.1126/science.adf4272). The work was supported by Princeton University, the National Science Foundation (2207518), and the Sloan Foundation (FG-2022-19104).